To install:

npm install yaml
# or
yarn add yaml

To use:

import { parse, stringify } from 'yaml'
// or
import YAML from 'yaml'
// or
const YAML = require('yaml')

yaml is a definitive library for YAML, the human friendly data serialization standard. This library:

The library is released under the ISC open source license, and the code is available on GitHub. It has no external dependencies and runs on Node.js as well as modern browsers.

For the purposes of versioning, any changes that break any of the endpoints or APIs documented here will be considered semver-major breaking changes. Undocumented library internals may change between minor versions, and previous APIs may be deprecated (but not removed).

The minimum supported TypeScript version of the included typings is 3.9; for use in earlier versions you may need to set skipLibCheck: true in your config. This requirement may be updated between minor versions of the library.

Note: These docs are for yaml@2. For v1, see the v1.10.0 tag for the source and for the documentation.

API Overview

The API provided by yaml has three layers, depending on how deep you need to go: Parse & Stringify, Documents, and the underlying Lexer/Parser/Composer. The first has the simplest API and "just works", the second gets you all the bells and whistles supported by the library along with a decent AST, and the third lets you get progressively closer to YAML source, if that's your thing.

A command-line tool is also included.

Parse & Stringify

import { parse, stringify } from 'yaml'


import {
} from 'yaml'

Content Nodes

import {
  isAlias, isCollection, isMap, isNode,
  isPair, isScalar, isSeq, Scalar,
  visit, visitAsync, YAMLMap, YAMLSeq
} from 'yaml'

Parsing YAML

import { Composer, Lexer, Parser } from 'yaml'

Parse & Stringify

# file.yml
  - A human-readable data serialization language
  - A complete JavaScript implementation

At its simplest, you can use YAML.parse(str) and YAML.stringify(value) just as you'd use JSON.parse(str) and JSON.stringify(value). If that's enough for you, everything else in these docs is really just implementation details.


import fs from 'fs'
import YAML from 'yaml'

// 3.14159

YAML.parse('[ true, false, maybe, null ]\n')
// [ true, false, 'maybe', null ]

const file = fs.readFileSync('./file.yml', 'utf8')
// { YAML:
//   [ 'A human-readable data serialization language',
//     '' ],
//   yaml:
//   [ 'A complete JavaScript implementation',
//     '' ] }

YAML.parse(str, reviver?, options = {}): any

str should be a string with YAML formatting. If defined, the reviver function follows the JSON implementation. See Options for more information on the last argument, an optional configuration object.

The returned value will match the type of the root value of the parsed YAML document, so Maps become objects, Sequences arrays, and scalars result in nulls, booleans, numbers and strings.

YAML.parse may throw on error, and it may log warnings using console.warn. It only supports input consisting of a single YAML document; for multi-document support you should use YAML.parseAllDocuments.


// '3.14159\n'

YAML.stringify([true, false, 'maybe', null])
// `- true
// - false
// - maybe
// - null
// `

YAML.stringify({ number: 3, plain: 'string', block: 'two\nlines\n' })
// `number: 3
// plain: string
// block: >
//   two
//   lines
// `

YAML.stringify(value, replacer?, options = {}): string

value can be of any type. The returned string will always include \n as the last character, as is expected of YAML documents. If defined, the replacer array or function follows the JSON implementation. See Options for more information on the last argument, an optional configuration object. For JSON compatibility, using a number or a string as the options value will set the indent option accordingly.

As strings in particular may be represented in a number of different styles, the simplest option for the value in question will always be chosen, depending mostly on the presence of escaped or control characters and leading & trailing whitespace.

To create a stream of documents, you may call YAML.stringify separately for each document's value, and concatenate the documents with the string ...\n as a separator.


import { parse, stringify } from 'yaml'

parse('number: 999')
// { number: 999 }

parse('number: 999', { intAsBigInt: true })
// { number: 999n }

parse('number: 999', { schema: 'failsafe' })
// { number: '999' }

The options supported by various yaml features are split into various categories, depending on how and where they are used. Options in various categories do not overlap, so it's fine to use a single "bag" of options and pass it to each function or method.

Parse Options

Parse options affect the parsing and composition of a YAML Document from it source.

Used by: parse(), parseDocument(), parseAllDocuments(), new Composer(), and new Document()

Name Type Default Description
intAsBigInt boolean false Whether integers should be parsed into BigInt rather than number values.
keepSourceTokens boolean false Include a srcToken value on each parsed Node, containing the CST token that was composed into this node.
lineCounter LineCounter If set, newlines will be tracked, to allow for lineCounter.linePos(offset) to provide the { line, col } positions within the input.
prettyErrors boolean true Include line/col position in errors, along with an extract of the source string.
strict boolean true When parsing, do not ignore errors required by the YAML 1.2 spec, but caused by unambiguous content.
uniqueKeys boolean ⎮ (a, b) => boolean true Whether key uniqueness is checked, or customised. If set to be a function, it will be passed two parsed nodes and should return a boolean value indicating their equality.

Document Options

Document options are relevant for operations on the Document object, which makes them relevant for both conversion directions.

Used by: parse(), parseDocument(), parseAllDocuments(), stringify(), new Composer(), and new Document()

Name Type Default Description
logLevel 'warn' ⎮ 'error' ⎮ 'silent' 'warn' Control the verbosity of parse(). Set to 'error' to silence warnings, and to 'silent' to also silence most errors (not recommended).
version '1.1' ⎮ '1.2' '1.2' The YAML version used by documents without a %YAML directive.

By default, the library will emit warnings as required by the YAML spec during parsing. If you'd like to silence these, set the logLevel option to 'error'.

Schema Options

parse('3') // 3 (Using YAML 1.2 core schema by default)
parse('3', { schema: 'failsafe' }) // '3'

parse('No') // 'No'
parse('No', { schema: 'json' }) // SyntaxError: Unresolved plain scalar "No"
parse('No', { schema: 'yaml-1.1' }) // false
parse('No', { version: '1.1' }) // false

Schema options determine the types of values that the document supports.

Aside from defining the language structure, the YAML 1.2 spec defines a number of different schemas that may be used. The default is the core schema, which is the most common one. The json schema is effectively the minimum schema required to parse JSON; both it and the core schema are supersets of the minimal failsafe schema.

The yaml-1.1 schema matches the more liberal YAML 1.1 types (also used by YAML 1.0), including binary data and timestamps as distinct tags. This schema accepts a greater variance in scalar values (with e.g. 'No' being parsed as false rather than a string value). The !!value and !!yaml types are not supported.

Used by: parse(), parseDocument(), parseAllDocuments(), stringify(), new Composer(), new Document(), and doc.setSchema()

Name Type Default Description
compat string ⎮ Tag[] ⎮ null null When parsing, warn about compatibility issues with the given schema. When stringifying, use scalar styles that are parsed correctly by the compat schema as well as the actual schema.
customTags Tag[] ⎮ function Array of additional tags to include in the schema
merge boolean 1.1: true 1.2: false Enable support for << merge keys. Default value depends on YAML version.
resolveKnownTags boolean true When using the 'core' schema, support parsing values with these explicit YAML 1.1 tags: !!binary, !!omap, !!pairs, !!set, !!timestamp. By default true.
schema string 1.1: 'yaml-1.1' 1.2: 'core' The base schema to use. Default value depends on YAML version. Built-in support is provided for 'core', 'failsafe', 'json', and 'yaml-1.1'. If using another value, customTags must be an array of tags.
sortMapEntries boolean ⎮ (a, b: Pair) => number false When stringifying, sort map entries. If true, sort by comparing key values using the native less-than < operator.
toStringDefaults ToStringOptions Override default values for toString() options.
const src = `
  source: &base { a: 1, b: 2 }
    <<: *base
    b: base`
const mergeResult = parse(src, { merge: true })
// { a: 1, b: 'base' }

Merge keys are a YAML 1.1 feature that is not a part of the 1.2 spec. To use a merge key, assign a map or its alias or an array of such as the value of a << key in a mapping. Multiple merge keys may be used on the same map, with earlier values taking precedence over latter ones, in case both define a value for the same key.

CreateNode Options

Used by: stringify(), new Document(), doc.createNode(), and doc.createPair()

Name Type Default Description
aliasDuplicateObjects boolean true During node construction, use anchors and aliases to keep strictly equal non-null objects as equivalent in YAML.
anchorPrefix string 'a' Default prefix for anchors, resulting in anchors a1, a2, ... by default.
flow boolean false Force the top-level collection node to use flow style.
keepUndefined boolean false Keep undefined object values when creating mappings and return a Scalar node when stringifying undefined.
tag string Specify the top-level collection type, e.g. '!!omap'. Note that this requires the corresponding tag to be available in this document's schema.

ToJS Options

parse('{[1, 2]: many}') // { '[ 1, 2 ]': 'many' }
parse('{[1, 2]: many}', { mapAsMap: true }) // Map { [ 1, 2 ] => 'many' }

These options influence how the document is transformed into "native" JavaScript representation.

Used by: parse(), doc.toJS() and node.toJS()

Name Type Default Description
mapAsMap boolean false Use Map rather than Object to represent mappings.
maxAliasCount number 100 Prevent exponential entity expansion attacks by limiting data aliasing; set to -1 to disable checks; 0 disallows all alias nodes.
onAnchor (value: any, count: number) => void Optional callback for each aliased anchor in the document.
reviver (key: any, value: any) => any Optionally apply a reviver function to the output, following the JSON specification but with appropriate extensions for handling Map and Set.

ToString Options

  { this: null, that: 'value' },
  { defaultStringType: 'QUOTE_SINGLE', nullStr: '~' }
// 'this': ~
// 'that': 'value'

The doc.toString() method may be called with additional options to control the resulting YAML string representation of the document.

Used by: stringify() and doc.toString()

Name Type Default Description
blockQuote boolean ⎮ 'folded' ⎮ 'literal' true Use block quote styles for scalar values where applicable. Set to false to disable block quotes completely.
collectionStyle 'any' ⎮ 'block' ⎮ 'flow' 'any' Enforce 'block' or 'flow' style on maps and sequences. By default, allows each collection to set its own flow: boolean property.
commentString (comment: string) => string Output should be valid for the current schema. By default, empty comment lines are left empty, lines consisting of a single space are replaced by #, and all other lines are prefixed with a #.
defaultKeyType 'BLOCK_FOLDED' ⎮ 'BLOCK_LITERAL' ⎮ 'QUOTE_DOUBLE' ⎮ 'QUOTE_SINGLE' ⎮ 'PLAIN' ⎮ null null If not null, overrides defaultStringType for implicit key values.
defaultStringType 'BLOCK_FOLDED' ⎮ 'BLOCK_LITERAL' ⎮ 'QUOTE_DOUBLE' ⎮ 'QUOTE_SINGLE' ⎮ 'PLAIN' 'PLAIN' The default type of string literal used to stringify values.
directives boolean ⎮ null null Include directives in the output. If true, at least the document-start marker --- is always included. If false, no directives or marker is ever included. If null, directives and marker may be included if required.
doubleQuotedAsJSON boolean false Restrict double-quoted strings to use JSON-compatible syntax.
doubleQuotedMinMultiLineLength number 40 Minimum length for double-quoted strings to use multiple lines to represent the value instead of escaping newlines.
falseStr string 'false' String representation for false values.
flowCollectionPadding boolean true When true, a single space of padding will be added inside the delimiters of non-empty single-line flow collections.
indent number 2 The number of spaces to use when indenting code. Should be a strictly positive integer.
indentSeq boolean true Whether block sequences should be indented.
lineWidth number 80 Maximum line width (set to 0 to disable folding). This is a soft limit, as only double-quoted semantics allow for inserting a line break in the middle of a word.
minContentWidth number 20 Minimum line width for highly-indented content (set to 0 to disable).
nullStr string 'null' String representation for null values.
simpleKeys boolean false Require keys to be scalars and always use implicit rather than explicit notation.
singleQuote boolean ⎮ null null Use 'single quote' rather than "double quote" where applicable. Set to false to disable single quotes completely.
trueStr string 'true' String representation for true values.


In order to work with YAML features not directly supported by native JavaScript data types, such as comments, anchors and aliases, yaml provides the Document API.

Parsing Documents

import fs from 'fs'
import { parseAllDocuments, parseDocument } from 'yaml'

const file = fs.readFileSync('./file.yml', 'utf8')
const doc = parseDocument(file)
// YAMLMap {
//   items:
//    [ Pair {
//        key: Scalar { value: 'YAML', range: [ 0, 4, 4 ] },
//        value:
//         YAMLSeq {
//           items:
//            [ Scalar {
//                value: 'A human-readable data serialization language',
//                range: [ 10, 54, 55 ] },
//              Scalar {
//                value: '',
//                range: [ 59, 93, 94 ] } ],
//           range: [ 8, 94, 94 ] } },
//      Pair {
//        key: Scalar { value: 'yaml', range: [ 94, 98, 98 ] },
//        value:
//         YAMLSeq {
//           items:
//            [ Scalar {
//                value: 'A complete JavaScript implementation',
//                range: [ 104, 140, 141 ] },
//              Scalar {
//                value: '',
//                range: [ 145, 180, 180 ] } ],
//           range: [ 102, 180, 180 ] } } ],
//   range: [ 0, 180, 180 ] }

These functions should never throw, provided that str is a string and the options are valid. Errors and warnings are included in the documents' errors and warnings arrays. In particular, if errors is not empty it's likely that the document's parsed contents are not entirely correct.

The contents of a parsed document will always consist of Scalar, Map, Seq or null values.

parseDocument(str, options = {}): Document

Parses a single Document from the input str; used internally by parse. Will include an error if str contains more than one document. See Options for more information on the second parameter.

parseAllDocuments(str, options = {}): Document[]

When parsing YAML, the input string str may consist of a stream of documents separated from each other by ... document end marker lines. parseAllDocuments will return an array of Document objects that allow these documents to be parsed and manipulated with more control. See Options for more information on the second parameter.

Creating Documents

new Document(value, replacer?, options = {})

Creates a new document. If value is defined, the document contents are initialised with that value, wrapped recursively in appropriate content nodes. If value is undefined, the document's contents is initialised as null. If defined, a replacer may filter or modify the initial document contents, following the same algorithm as the JSON implementation. See Options for more information on the last argument.

Member Type Description
commentBefore string? A comment at the very beginning of the document. If not empty, separated from the rest of the document by a blank line or the doc-start indicator when stringified.
comment string? A comment at the end of the document. If not empty, separated from the rest of the document by a blank line when stringified.
contents Node ⎮ any The document contents.
directives Directives Controls for the %YAML and %TAG directives, as well as the doc-start marker ---.
errors Error[] Errors encountered during parsing.
schema Schema The schema used with the document.
warnings Error[] Warnings encountered during parsing.
import { Document } from 'yaml'

const doc = new Document(['some', 'values', { balloons: 99 }])
doc.commentBefore = ' A commented document'

// # A commented document
// - some
// - values
// - balloons: 99

The Document members are all modifiable, though it's unlikely that you'll have reason to change errors, schema or warnings. In particular you may be interested in both reading and writing contents. Although parseDocument() and parseAllDocuments() will leave it with YAMLMap, YAMLSeq, Scalar or null contents, it can be set to anything.

Document Methods

Method Returns Description
clone() Document Create a deep copy of this Document and its contents. Custom Node values that inherit from Object still refer to their original instances.
createAlias(node: Node, name?: string) Alias Create a new Alias node, adding the required anchor for node. If name is empty, a new anchor name will be generated.
createNode(value, options?) Node Recursively wrap any input with appropriate Node containers. See Creating Nodes for more information.
createPair(key, value, options?) Pair Recursively wrap key and value into a Pair object. See Creating Nodes for more information.
setSchema(version, options?) void Change the YAML version and schema used by the document. version must be either '1.1' or '1.2'; accepts all Schema options.
toJS(options?) any A plain JavaScript representation of the document contents.
toJSON() any A JSON representation of the document contents.
toString(options?) string A YAML representation of the document.
const doc = parseDocument('a: 1\nb: [2, 3]\n')
doc.get('a') // 1
doc.getIn([]) // YAMLMap { items: [Pair, Pair], ... }
doc.hasIn(['b', 0]) // true
doc.addIn(['b'], 4) // -> doc.get('b').items.length === 3
doc.deleteIn(['b', 1]) // true
doc.getIn(['b', 1]) // 4

In addition to the above, the document object also provides the same accessor methods as collections, based on the top-level collection: add, delete, get, has, and set, along with their deeper variants addIn, deleteIn, getIn, hasIn, and setIn. For the *In methods using an empty path value (i.e. null, undefined, or []) will refer to the document's top-level contents.

Document#toJS(), Document#toJSON() and Document#toString()

const src = '1969-07-21T02:56:15Z'
const doc = parseDocument(src, { customTags: ['timestamp'] })

// Date { 1969-07-21T02:56:15.000Z }

// '1969-07-21T02:56:15.000Z'

// '1969-07-21T02:56:15\n'

For a plain JavaScript representation of the document, toJS(options = {}) is your friend. Its output may include Map and Set collections (e.g. if the mapAsMap option is true) and complex scalar values like Date for !!timestamp, but all YAML nodes will be resolved. See Options for more information on the optional parameter.

For a representation consisting only of JSON values, use toJSON().

To stringify a document as YAML, use toString(options = {}). This will also be called by String(doc) (with no options). This method will throw if the errors array is not empty. See Options for more information on the optional parameter.

Stream Directives

const doc = new Document()
> {
    docStart: null, // set true to force the doc-start marker
    docEnd: false, // set true to force the doc-end marker
    tags: { '!!': ',2002:' }, // Record<handle, prefix>
    yaml: { explicit: false, version: '1.2' }

A YAML document may be preceded by %YAML and %TAG directives; their state is accessible via the directives member of a Document. After parsing or other creation, the contents of doc.directives are mutable, and will influence the YAML string representation of the document.

The contents of doc.directives.tags are used both for the %TAG directives and when stringifying tags within the document. Each of the handles must start and end with a ! character; ! is by default the local tag and !! is used for default tags. See the section on custom tags for more on this topic.

doc.contents.yaml determines if an explicit %YAML directive should be included in the output, and what version it should use. If changing the version after the document's creation, you'll probably want to use doc.setSchema() as it will also update the schema accordingly.

Content Nodes

After parsing, the contents value of each YAML.Document is the root of an Abstract Syntax Tree of nodes representing the document (or null for an empty document).

Both scalar and collection values may have an anchor associated with them; this is rendered in the string representation with a & prefix, so e.g. in foo: &aa bar, the value bar has the anchor aa. Anchors are used by Alias nodes to allow for the same value to be used in multiple places in the document. It is valid to have an anchor associated with a node even if it has no aliases.

Scalar Values

class NodeBase {
  comment?: string        // a comment on or immediately after this
  commentBefore?: string  // a comment before this
  range?: [number, number, number]
      // The `[start, value-end, node-end]` character offsets for the part
      // of the source parsed into this node (undefined if not parsed).
      // The `value-end` and `node-end` positions are themselves not
      // included in their respective ranges.
  spaceBefore?: boolean
      // a blank line before this node and its commentBefore
  tag?: string       // a fully qualified tag, if required
  clone(): NodeBase  // a copy of this node
  toJS(doc, options?): any // a plain JS representation of this node
  toJSON(): any      // a plain JSON representation of this node

For scalar values, the tag will not be set unless it was explicitly defined in the source document; this also applies for unsupported tags that have been resolved using a fallback tag (string, YAMLMap, or YAMLSeq).

class Scalar<T = unknown> extends NodeBase {
  anchor?: string  // an anchor associated with this node
  format?: 'BIN' | 'HEX' | 'OCT' | 'TIME' | undefined
      // By default (undefined), numbers use decimal notation.
      // The YAML 1.2 core schema only supports 'HEX' and 'OCT'.
    'QUOTE_DOUBLE' | 'QUOTE_SINGLE' | undefined
  value: T

A parsed document's contents will have all of its non-object values wrapped in Scalar objects, which themselves may be in some hierarchy of YAMLMap and YAMLSeq collections. However, this is not a requirement for the document's stringification, which is rather tolerant regarding its input values, and will use doc.createNode() when encountering an unwrapped value.

When stringifying, the node type will be taken into account by !!str and !!binary values, and ignored by other scalars. On the other hand, !!int and !!float stringifiers will take format into account.


class Pair<K = unknown, V = unknown> {
  key: K    // When parsed, key and value are always
  value: V  // Node or null, but can be set to anything

class Collection extends NodeBase {
  anchor?: string  // an anchor associated with this node
  flow?: boolean   // use flow style when stringifying this
  schema?: Schema
  addIn(path: Iterable<unknown>, value: unknown): void
  clone(schema?: Schema): NodeBase  // a deep copy of this collection
  deleteIn(path: Iterable<unknown>): boolean
  getIn(path: Iterable<unknown>, keepScalar?: boolean): unknown
  hasIn(path: Iterable<unknown>): boolean
  setIn(path: Iterable<unknown>, value: unknown): void

class YAMLMap<K = unknown, V = unknown> extends Collection {
  items: Pair<K, V>[]
  add(pair: Pair<K, V> | { key: K; value: V }, overwrite?: boolean): void
  delete(key: K): boolean
  get(key: K, keepScalar?: boolean): unknown
  has(key: K): boolean
  set(key: K, value: V): void

class YAMLSeq<T = unknown> extends Collection {
  items: T[]
  add(value: T): void
  delete(key: number | Scalar<number>): boolean
  get(key: number | Scalar<number>, keepScalar?: boolean): unknown
  has(key: number | Scalar<number>): boolean
  set(key: number | Scalar<number>, value: T): void

Within all YAML documents, two forms of collections are supported: sequential YAMLSeq collections and key-value YAMLMap collections. The JavaScript representations of these collections both have an items array, which may (YAMLSeq) or must (YAMLMap) consist of Pair objects that contain a key and a value of any type, including null. The items array of a YAMLSeq object may contain values of any type.

When stringifying collections, by default block notation will be used. Flow notation will be selected if flow is true, the collection is within a surrounding flow collection, or if the collection is in an implicit key.

The yaml-1.1 schema includes additional collections that are based on YAMLMap and YAMLSeq: OMap and Pairs are sequences of Pair objects (OMap requires unique keys & corresponds to the JS Map object), and Set is a map of keys with null values that corresponds to the JS Set object.

All of the collections provide the following accessor methods:

Method Returns Description
add(value), addIn(path, value) void Adds a value to the collection. For !!map and !!omap the value must be a Pair instance or a { key, value } object, which may not have a key that already exists in the map.
delete(key), deleteIn(path) boolean Removes a value from the collection. Returns true if the item was found and removed.
get(key, [keep]), getIn(path, [keep]) any Returns value at key, or undefined if not found. By default unwraps scalar values from their surrounding node; to disable set keep to true (collections are always returned intact).
has(key), hasIn(path) boolean Checks if the collection includes a value with the key key.
set(key, value), setIn(path, value) any Sets a value in this collection. For !!set, value needs to be a boolean to add/remove the item from the set. When overwriting a Scalar value with a scalar, the original node is retained.
const doc = new YAML.Document({ a: 1, b: [2, 3] }) // { a: 1, b: [ 2, 3 ] }
doc.add({ key: 'c', value: 4 }) // { a: 1, b: [ 2, 3 ], c: 4 }
doc.addIn(['b'], 5)             // { a: 1, b: [ 2, 3, 5 ], c: 4 }
doc.set('c', 42)                // { a: 1, b: [ 2, 3, 5 ], c: 42 }
doc.setIn(['c', 'x']) // Error: Expected YAML collection at c. Remaining path: x
doc.delete('c')                 // { a: 1, b: [ 2, 3, 5 ] }
doc.deleteIn(['b', 1])          // { a: 1, b: [ 2, 5 ] }

doc.get('a') // 1
doc.get('a', true) // Scalar { value: 1 }
doc.getIn(['b', 1]) // 5
doc.has(doc.createNode('a')) // true
doc.has('c') // false
doc.hasIn(['b', '0']) // true

For all of these methods, the keys may be nodes or their wrapped scalar values (i.e. 42 will match Scalar { value: 42 }). Keys for !!seq should be positive integers, or their string representations. add() and set() do not automatically call doc.createNode() to wrap the value.

Each of the methods also has a variant that requires an iterable as the first parameter, and allows fetching or modifying deeper collections. If any intermediate node in path is a scalar rather than a collection, an error will be thrown. If any of the intermediate collections is not found:

Note that for addIn the path argument points to the collection rather than the item; for maps its value should be a Pair or an object with { key, value } fields.

Alias Nodes

class Alias extends NodeBase {
  source: string
  resolve(doc: Document): Scalar | YAMLMap | YAMLSeq | undefined

const obj = YAML.parse('[ &x { X: 42 }, Y, *x ]')
  // => [ { X: 42 }, 'Y', { X: 42 } ]
obj[2].Z = 13
  // => [ { X: 42, Z: 13 }, 'Y', { X: 42, Z: 13 } ]
  // - &a1
  //   X: 42
  //   Z: 13
  // - Y
  // - *a1

Alias nodes provide a way to include a single node in multiple places in a document; the source of an alias node must be a preceding anchor in the document. Circular references are fully supported, and where possible the JS representation of alias nodes will be the actual source object. For ease of use, alias nodes also provide a resolve(doc) method to dereference its source node.

When nodes are constructed from JS structures (e.g. during YAML.stringify()), multiple references to the same object will result in including an autogenerated anchor at its first instance, and alias nodes to that anchor at later references.

Creating Nodes

const doc = new YAML.Document(['some', 'values'])
// Document {
//   contents:
//     YAMLSeq {
//       items:
//        [ Scalar { value: 'some' },
//          Scalar { value: 'values' } ] } }

const map = doc.createNode({ balloons: 99 })
// YAMLMap {
//   items:
//    [ Pair {
//        key: Scalar { value: 'balloons' },
//        value: Scalar { value: 99 } } ] }

doc.get(0, true).comment = ' A commented item'
// - some # A commented item
// - values
// - balloons: 99

doc.createNode(value, replacer?, options?): Node

To create a new node, use the createNode(value, options?) document method. This will recursively wrap any input with appropriate Node containers. Generic JS Object values as well as Map and its descendants become mappings, while arrays and other iterable objects result in sequences. With Object, entries that have an undefined value are dropped.

If value is already a Node instance, it will be directly returned. To create a copy of a node, use instead the node.clone() method. For collections, the method accepts a single Schema argument, which allows overwriting the original's schema value.

Use a replacer to apply a replacer array or function, following the JSON implementation. To force flow styling on a collection, use the flow: true option. For all available options, see the CreateNode Options section.

The primary purpose of this method is to enable attaching comments or other metadata to a value, or to otherwise exert more fine-grained control over the stringified output. To that end, you'll need to assign its return value to the contents of a document (or somewhere within said contents), as the document's schema is required for YAML string output. If you're not interested in working with such metadata, document contents may also include non-Node values at any level.

doc.createAlias(node, name?): Alias

const alias = doc.createAlias(doc.get(1, true), 'foo')
// - some # A commented item
// - &foo values
// - balloons: 99
// - *foo

Create a new Alias node, ensuring that the target node has the required anchor. If node already has an anchor, name is ignored. Otherwise, the node.anchor value will be set to name, or if an anchor with that name is already present in the document, name will be used as a prefix for a new unique anchor. If name is undefined, the generated anchor will use 'a' as a prefix.

You should make sure to only add alias nodes to the document after the nodes to which they refer, or the document's YAML stringification will fail.

new YAMLMap(), new YAMLSeq(), doc.createPair(key, value): Pair

import { Document, YAMLSeq } from 'yaml'

const doc = new Document(new YAMLSeq())
doc.contents.items = [
  'some values',
  { including: 'objects', 3: 'a string' }
doc.add(doc.createPair(1, 'a number'))

// - some values
// - 42
// - "3": a string
//   including: objects
// - 1: a number

To construct a YAMLSeq or YAMLMap, use new Document() or doc.createNode() with array, object or iterable input, or create the collections directly by importing the classes from yaml.

Once created, normal array operations may be used to modify the items array. New Pair objects may created either by importing the class from yaml and using its new Pair(key, value) constructor, or by using the doc.createPair(key, value, options?) method. The latter will recursively wrap the key and value as nodes, and accepts the same options as doc.createNode()

Finding and Modifying Nodes

const doc = YAML.parseDocument(`
  - some values
  - 42
  - "3": a string
    including: objects
  - 1: a number

const obs = doc.getIn([2, 'including'], true)
obs.type = 'QUOTE_DOUBLE'

YAML.visit(doc, {
  Pair(_, pair) {
    if (pair.key && pair.key.value === '3') return YAML.visit.REMOVE
  Scalar(key, node) {
    if (
      key !== 'key' &&
      typeof node.value === 'string' &&
      node.type === 'PLAIN'
    ) {
      node.type = 'QUOTE_SINGLE'

// - 'some values'
// - 42
// - including: "objects"
// - 1: 'a number'

In general, it's safe to modify nodes manually, e.g. splicing the items array of a YAMLMap or setting its flow value to true. For operations on nodes at a known location in the tree, it's probably easiest to use doc.getIn(path, true) to access them. For more complex or general operations, a visitor API is provided:

YAML.visit(node, visitor): void

Apply a visitor to an AST node or document.

Walks through the tree (depth-first) starting from node, calling a visitor function with three arguments:

The return value of the visitor may be used to control the traversal:

If visitor is a single function, it will be called with all values encountered in the tree, including e.g. null values. Alternatively, separate visitor functions may be defined for each Map, Pair, Seq, Alias and Scalar node. To define the same visitor function for more than one node type, use the Collection (map and seq), Value (map, seq & scalar) and Node (alias, map, seq & scalar) targets. Of all these, only the most specific defined one will be used for each node.

YAML.visitAsync(node, visitor): Promise<void>

The same as visit(), but allows for visitor functions that return a promise which resolves to one of the above-defined control values.

Identifying Node Types

import {
  isCollection, // map or seq
  isNode, // alias, scalar, map or seq
} from 'yaml'

const doc = new Document({ foo: [13, 42] })
isDocument(doc) === true
isNode(doc) === false
isMap(doc.contents) === true
isNode(doc.contents) === true
isPair(doc.contents.items[0]) === true
isCollection(doc.get('foo')) === true
isScalar(doc.getIn(['foo', 1])) === true

isAlias(x: unknown): boolean

isCollection(x: unknown): boolean

isDocument(x: unknown): boolean

isMap(x: unknown): boolean

isNode(x: unknown): boolean

isPair(x: unknown): boolean

isScalar(x: unknown): boolean

isSeq(x: unknown): boolean

To find out what you've got, a family of custom type guard functions is provided. These should be preferred over other methods such as instanceof checks, as they'll work even if the nodes have been created by a different instance of the library.

Internally, node identification uses property symbols that are set on instances during their construction.

Comments and Blank Lines

const doc = YAML.parseDocument(`
# This is YAML.
it has:

  - an array

  - of values

doc.toJS() // { 'it has': [ 'an array', 'of values' ] }
doc.commentBefore // ' This is YAML.'

const seq = doc.get('it has')
seq.spaceBefore // true

seq.items[0].comment = ' item comment'
seq.comment = ' collection end comment'

// # This is YAML.
// it has:
//   - an array # item comment
//   - of values
//   # collection end comment

A primary differentiator between this and other YAML libraries is the ability to programmatically handle comments, which according to the spec "must not have any effect on the serialization tree or representation graph. In particular, comments are not associated with a particular node." Similarly to comments, the YAML spec instructs non-content blank lines to be discarded.

This library does allow comments and blank lines to be handled programmatically, and does attach them to particular nodes (most often, the following node). Each Scalar, Map, Seq and the Document itself has comment, commentBefore members that may be set to a stringifiable value, and a spaceBefore boolean to add an empty line before the comment.

The string contents of comments are not processed by the library, except for merging adjacent comment and blank lines together. Document comments will be separated from the rest of the document by a blank line. In the node member values, comment lines terminating with the # indicator are represented by a single space, while completely empty lines are represented as empty strings.

Scalar block values with "keep" chomping (i.e. with + in their header) consider any trailing empty lines to be a part of their content, so the following node's spaceBefore or commentBefore with leading whitespace is ignored.

Note: Due to implementation details, the library's comment handling is not completely stable, in particular for trailing comments. When creating, writing, and then reading a YAML file, comments may sometimes be associated with a different node.

Custom Data Types

import { parse, parseDocument } from 'yaml'

parse('2001-12-15 2:59:43')
// '2001-12-15 2:59:43'

parse('!!timestamp 2001-12-15 2:59:43')
// 2001-12-15T02:59:43.000Z (Date instance)

const doc = parseDocument('2001-12-15 2:59:43', { customTags: ['timestamp'] })
// 'Sat Dec 15 2001'

The easiest way to extend a schema is by defining the additional tags that you wish to support. To do that, the customTags option allows you to provide an array of custom tag objects or tag identifiers. In particular, the built-in tags that are a part of the core and yaml-1.1 schemas may be referred to by their string identifiers. For those tags that are available in both, only the core variant is provided as a custom tag.

For further customisation, customTags may also be a function (Tag[]) => (Tag[]) that may modify the schema's base tag array.

Some additional data types are available separately via the yaml-types package, including support for:

Built-in Custom Tags

parse('[ one, true, 42 ]')
// [ 'one', true, 42 ]

parse('[ one, true, 42 ]', { schema: 'failsafe' })
// [ 'one', 'true', '42' ]

parse('[ one, true, 42 ]', { schema: 'failsafe', customTags: ['int'] })
// [ 'one', 'true', 42 ]

YAML 1.2 Core Schema

These tags are a part of the YAML 1.2 Core Schema, and may be useful when constructing a parser or stringifier for a more limited set of types, based on the failsafe schema. Some of these define a format value; this will be added to the parsed nodes and affects the node's stringification.

If including more than one custom tag from this set, make sure that the 'float' and 'int' tags precede any of the other !!float and !!int tags.

Identifier Regular expression YAML Type Format Example values
'bool' true⎮True⎮TRUE⎮false⎮False⎮FALSE !!bool true, false
'float' [-+]?(0⎮[1-9][0-9]*)\.[0-9]* !!float 4.2, -0.0
'floatExp' [-+]?(0⎮[1-9][0-9]*)(\.[0-9]*)?[eE][-+]?[0-9]+ !!float 'EXP' 4.2e9
'floatNaN' [-+]?(\.inf⎮\.Inf⎮\.INF)⎮\.nan⎮\.NaN⎮\.NAN !!float -Infinity
'int' [-+]?[0-9]+ !!int 42, -0
'intHex' 0x[0-9a-fA-F]+ !!int 'HEX' 0xff0033
'intOct' 0o[0-7]+ !!int 'OCT' 0o127
'null' ~⎮null⎮Null⎮NULL !!null null

YAML 1.1

These tags are a part of the YAML 1.1 language-independent types, but are not a part of any default YAML 1.2 schema.

Identifier YAML Type JS Type Description
'binary' !!binary Uint8Array Binary data, represented in YAML as base64 encoded characters.
'floatTime' !!float Number Sexagesimal floating-point number format, e.g. 190:20:30.15. To stringify with this tag, the node format must be 'TIME'.
'intTime' !!int Number Sexagesimal integer number format, e.g. 190:20:30. To stringify with this tag, the node format must be 'TIME'.
'omap' !!omap Map Ordered sequence of key: value pairs without duplicates. Using mapAsMap: true together with this tag is not recommended, as it makes the parse → stringify loop non-idempotent.
'pairs' !!pairs Array Ordered sequence of key: value pairs allowing duplicates. To create from JS, use doc.createNode(array, { tag: '!!pairs' }).
'set' !!set Set Unordered set of non-equal values.
'timestamp' !!timestamp Date A point in time, e.g. 2001-12-15T02:59:43.

Writing Custom Tags

import { YAMLMap, stringify } from 'yaml'
import { stringifyString } from 'yaml/util'

const regexp = {
  identify: value => value instanceof RegExp,
  tag: '!re',
  resolve(str) {
    const match = str.match(/^\/([\s\S]+)\/([gimuy]*)$/)
    if (!match) throw new Error('Invalid !re value')
    return new RegExp(match[1], match[2])

const sharedSymbol = {
  identify: value => value?.constructor === Symbol,
  tag: '!symbol/shared',
  resolve: str => Symbol.for(str),
  stringify(item, ctx, onComment, onChompKeep) {
    const key = Symbol.keyFor(item.value)
    if (key === undefined) throw new Error('Only shared symbols are supported')
    return stringifyString({ value: key }, ctx, onComment, onChompKeep)

class YAMLNullObject extends YAMLMap {
  tag = '!nullobject'
  toJSON(_, ctx) {
    const obj = super.toJSON(_, { ...ctx, mapAsMap: false }, Object)
    return Object.assign(Object.create(null), obj)

const nullObject = {
  tag: '!nullobject',
  collection: 'map',
  nodeClass: YAMLNullObject,
  identify: v => !!(typeof v === 'object' && v && !Object.getPrototypeOf(v))

// slightly more complicated object type
class YAMLError extends YAMLMap {
  tag = '!error'
  toJSON(_, ctx) {
    const { name, message, stack, } = super.toJSON(
      { ...ctx, mapAsMap: false },
    // craft the appropriate error type
    const Cls =
      name === 'EvalError'
        ? EvalError
        : name === 'RangeError'
          ? RangeError
          : name === 'ReferenceError'
            ? ReferenceError
            : name === 'SyntaxError'
              ? SyntaxError
              : name === 'TypeError'
                ? TypeError
                : name === 'URIError'
                  ? URIError
                  : Error
    if ( !== name) {
      Object.defineProperty(er, 'name', {
        value: name,
        enumerable: false,
        configurable: true
    Object.defineProperty(er, 'stack', {
      value: stack,
      enumerable: false,
      configurable: true
    return Object.assign(er, rest)

  static from(schema, obj, ctx) {
    const { name, message, stack } = obj
    // ensure these props remain, even if not enumerable
    return super.from(schema, { ...obj, name, message, stack }, ctx)

const error = {
  tag: '!error',
  collection: 'map',
  nodeClass: YAMLError,
  identify: v => !!(typeof v === 'object' && v && v instanceof Error)

    regexp: /foo/gi,
    symbol: Symbol.for('bar'),
    nullobj: Object.assign(Object.create(null), { a: 1, b: 2 }),
    error: new Error('This was an error')
  { customTags: [regexp, sharedSymbol, nullObject, error] }
// regexp: !re /foo/gi
// symbol: !symbol/shared bar
// nullobj: !nullobject
//   a: 1
//   b: 2
// error: !error
//   name: Error
//   message: 'This was an error'
//   stack: |
//     at some-file.js:1:3

In YAML-speak, a custom data type is represented by a tag. To define your own tag, you need to account for the ways that your data is both parsed and stringified. Furthermore, both of those processes are split into two stages by the intermediate AST node structure.

If you wish to implement your own custom tags, the !!binary and !!set tags as well as the yaml-types package provide relatively cohesive examples to study in addition to the simple examples in the sidebar here.

Custom collection types (ie, Maps, Sets, objects, and arrays; anything with child properties that may not be propertly serialized to a scalar value) may provide a nodeClass property that extends the YAMLMap and YAMLSeq classes, which will be used for parsing and stringifying objects with the specified tag.

Parsing Custom Data

At the lowest level, the Lexer and Parser will take care of turning string input into a concrete syntax tree (CST). In the CST all scalar values are available as strings, and maps & sequences as collections of nodes. Each schema includes a set of default data types, which handle converting at least strings, maps and sequences into their AST nodes. These are considered to have implicit tags, and are autodetected. Custom tags, on the other hand, should almost always define an explicit tag with which their value will be prefixed. This may be application-specific local !tag, a shorthand !ns!tag, or a verbatim !<,2019:tag>.

Once identified by matching the tag, the resolve(value, onError): Node | any function will turn a parsed value into an AST node. value may be either a string, a YAMLMap or a YAMLSeq, depending on the node's shape. A custom tag should verify that value is of its expected type.

Note that during the CST -> AST composition, the anchors and comments attached to each node are also resolved for each node. This metadata will unfortunately be lost when converting the values to JS objects, so collections should have values that extend one of the existing collection classes. Collections should therefore either fall back to their parent classes' toJSON() methods, or define their own in order to allow their contents to be expressed as the appropriate JS object.

Creating Nodes and Stringifying Custom Data

As with parsing, turning input data into its YAML string representation is a two-stage process as the input is first turned into an AST tree before stringifying it. This allows for metadata and comments to be attached to each node, and for e.g. circular references to be resolved. For scalar values, this means just wrapping the value within a Scalar class while keeping it unchanged.

As values may be wrapped within objects and arrays, doc.createNode() uses each tag's identify(value): boolean function to detect custom data types. For the same reason, collections need to define their own createNode(schema, value, ctx): Collection functions that may recursively construct their equivalent collection class instances.

Finally, stringify(item, ctx, ...): string defines how your data should be represented as a YAML string, in case the default stringifiers aren't enough. For collections in particular, the default stringifier should be perfectly sufficient. 'yaml/util' exports stringifyNumber(item) and stringifyString(item, ctx, ...), which may be of use for custom scalar data.

Custom Tag API

import {
  createNode, // (value, tagName, ctx) => Node -- Create a new node
  createPair, // (key, value, ctx) => Pair -- Create a new pair
  debug, // (logLevel, ...messages) => void -- Log debug messages to console
  findPair, // (items, key) => Pair? -- Given a key, find a matching Pair
  foldFlowLines, // (text, indent, mode, options) => string -- Fold long lines
  mapTag, // CollectionTag
  seqTag, // CollectionTag
  stringTag, // ScalarTag
  stringifyNumber, // (node) => string
  stringifyString, // (node, ctx, ...) => string
  toJS, // (value, arg, ctx) => any -- Recursively convert to plain JS
  warn // (logLevel, warning) => void -- Emit a warning
} from 'yaml/util'

To define your own tag, you'll need to define an object comprising of some of the following fields. Those in bold are required:

Parsing YAML

import {
} from 'yaml'

If you're interested only in the final output, parse() will directly produce native JavaScript If you'd like to retain the comments and other metadata, parseDocument() and parseAllDocuments() will produce Document instances that allow for further processing. If you're looking to do something more specific, this section might be for you.

Internally, the process of turning a sequence of characters into Documents relies on three stages, each of which is also exposed to external users. First, the Lexer splits the character stream into lexical tokens, i.e. sequences of characters and control codes. Next, the Parser builds concrete syntax tree representations of each document and directive in the stream. Finally, the Composer builds a more user-friendly and accessible Document representation of each document.

Both the Lexer and Parser accept incomplete input, allowing for them and the Composer to be used with e.g. Node.js streams or other systems that handle data in chunks.


import { Lexer } from 'yaml'

const tokens = new Lexer().lex('foo: bar\nfee:\n  [24,"42"]\n')
> [
    '\x02', '\x1F', 'foo',  ':',
    ' ',    '\x1F', 'bar',  '\n',
    '\x1F', 'fee',  ':',    '\n',
    '  ',   '[',    '\x1F', '24',
    ',',    '"42"', ']',    '\n'

new Lexer()

lexer.lex(src: string, incomplete?: boolean): Generator<string>

The API for the lexer is rather minimal, and offers no configuration. If the input stream is chunked, the lex() method may be called separately for each chunk if the incomplete argument is true. At the end of input, lex() should be called a final time with incomplete: false to ensure that the remaining tokens are emitted.

Internally, the lexer operates a state machine that determines how it parses its input. Initially, the lexer is always in the stream state. The lexer constructor and its lex() method should never throw an error.

All tokens are identifiable either by their exact value or their first character. In addition to slices of the input stream, a few control characters are additionally used within the output.

Value Token Meaning
\x02 doc-mode Start of a document within the default stream context.
\x18 flow-error-end Unexpected end of a flow collection, e.g. due to an unindent. Should be considered an error.
\x1f scalar The next token after this one is a scalar value, irrespective of its value or first character.
\n, \r\n newline In certain cases (such as end of input), an empty string may also be emitted; it should also be considered as a newline.
--- doc-start Explicit marker for the start of a document. Will be preceded by a doc-mode token.
... doc-end Explicit marker for the end of a document.
- seq-item-ind Block sequence item indicator, separated by whitespace.
? explicit-key-ind Explicit block map key indicator, separated by whitespace.
: map-value-ind Block map value indicator.
{ flow-map-start
} flow-map-end
[ flow-seq-start
] flow-seq-end
, comma Separator between flow collection items.
\u{FEFF} byte-order-mark Treated as whitespace in stream & content in a document.

If any of the control characters do show up directly in the input stream, they will be treated normally, and even when bare will be preceded by a SCALAR control token in the output.

All remaining tokens are identifiable by their first character:

First char Token Meaning
, \t space Only contains space characters if token indicates indentation. Otherwise may contain repeats of either character.
# comment Separated from preceding by whitespace. Does not include the trailing newline.
% directive-line Only produced in a stream context.
* alias
& anchor
! tag
' single-quoted-scalar Should also include ' as a last character, if input is valid.
" double-quoted-scalar Should also include " as a last character, if input is valid.
, > block-scalar-header Expected to be followed by optional whitespace & comment, a newline, and then a scalar value.


import { Parser } from 'yaml'

for (const token of new Parser().parse('foo: [24,"42"]\n'))
  console.dir(token, { depth: null })

> {
    type: 'document',
    offset: 0,
    start: [],
    value: {
      type: 'block-map',
      offset: 0,
      indent: 0,
      items: [
          start: [],
          key: { type: 'scalar', offset: 0, indent: 0, source: 'foo' },
          sep: [
            { type: 'map-value-ind', offset: 3, indent: 0, source: ':' },
            { type: 'space', offset: 4, indent: 0, source: ' ' }
          value: {
            type: 'flow-collection',
            offset: 5,
            indent: 0,
            start: { type: 'flow-seq-start', offset: 5, indent: 0, source: '[' },
            items: [
              { type: 'scalar', offset: 6, indent: 0, source: '24' },
              { type: 'comma', offset: 8, indent: 0, source: ',' },
                type: 'double-quoted-scalar',
                offset: 9,
                indent: 0,
                source: '"42"'
            end: [
              { type: 'flow-seq-end', offset: 13, indent: 0, source: ']' },
              { type: 'newline', offset: 14, indent: 0, source: '\n' }

The parser by default uses an internal Lexer instance, and provides a similarly minimal API for producing a Concrete Syntax Tree representation of the input stream.

The tokens emitted by the parser are JavaScript objects, each of which has a type value that's one of the following: directive-line, document, byte-order-mark, space, comment, newline. Of these, only directive-line and document should be considered as content.

The parser does not validate its output, trying instead to produce a most YAML-ish representation of any input. It should never throw errors, but may (rarely) include error tokens in its output.

To validate a CST, you will need to compose it into a Document. If the document contains errors, they will be included in the document's errors array, and each error will will contain an offset within the source string, which you may then use to find the corresponding node in the CST.

new Parser(onNewLine?: (offset: number) => void)

Create a new parser. If defined, onNewLine is called separately with the start position of each new line (in parse(), including the start of input).

parser.parse(source: string, incomplete = false): Generator<Token, void>

Parse source as a YAML stream, generating tokens for each directive, document and other structure as it is completely parsed. If incomplete, a part of the last line may be left as a buffer for the next call.

Errors are not thrown, but are yielded as { type: 'error', offset, message } tokens. string): Generator<Token, void>

Advance the parser by one lexical token. Used internally by parser.parse(); exposed to allow for use with an external lexer.

For debug purposes, if the LOG_TOKENS env var is true-ish, all lexical tokens will be pretty-printed using console.log() as they are being processed.

CST Nodes

For a complete description of CST node interfaces, please consult the cst.ts source.

Some of the most common node properties include:

Property Type Description
type string The only node property that's always defined. Identifies the node type. May be used as a TS type guard.
offset number The start index within the source string or character stream.
source string A raw string representation of the node's value, including all newlines and indentation.
indent number The indent level of the current line; mostly just for internal use.
items Item[] The contents of a collection; exact shape depends on the collection type.
start, sep, end SourceToken[] Content before, within, and after "actual" values. Includes item and collection indicators, anchors, tags, comments, as well as other things.

Collection items contain some subset of the following properties:

Item property Type Description
start SourceToken[] Always defined. Content before the actual value. May include comments that are later assigned to the preceding item.
key Token ⎮ null Set for key/value pairs only, so never used in block sequences.
sep SourceToken[] Content between the key and the value. If defined, indicates that the key logically exists, even if its value is null.
value Token ⎮ null The value. Normally set, but may be left out for e.g. explicit keys with no matching value.

Counting Lines

import { LineCounter, Parser } from 'yaml'

const lineCounter = new LineCounter()
const parser = new Parser(lineCounter.addNewLine))
const tokens = parser.parse('foo:\n- 24\n- "42"\n')
Array.from(tokens) // forces iteration

> [ 0, 5, 10, 17 ]
> { line: 1, col: 4 }
> { line: 2, col: 1 }

new LineCounter()

Tracks newlines during parsing in order to provide an efficient API for determining the one-indexed { line, col } position for any offset within the input.

lineCounter.addNewLine(offset: number)

Adds the starting index of a new line. Should be called in order, or the internal lineStarts array will need to be sorted before calling linePos(). Bound to the instance, so may be used directly as a callback.

lineCounter.linePos(offset: number): { line: number, col: number }

Performs a binary search and returns the 1-indexed { line, col } position of offset. If line === 0, addNewLine has never been called or offset is before the first known newline.


import { Composer, Parser } from 'yaml'

const src = 'foo: bar\nfee: [24, "42"]'
const tokens = new Parser().parse(src)
const docs = new Composer().compose(tokens)

Array.from(docs, doc => doc.toJS())
> [{ foo: 'bar', fee: [24, '42'] }]

new Composer(options?: ParseOptions & DocumentOptions & SchemaOptions)

Create a new Document composer. Does not include an internal Parser instance, so an external one will be needed. options will be used during composition, and passed to the new Document constructor.

composer.compose(tokens: Iterable<Token>, forceDoc?: boolean, endOffset?: number): Generator<Document.Parsed>

Compose tokens into documents. Convenience wrapper combining calls to and composer.end(). Token): Generator<Document.Parsed>

Advance the composed by one CST token.

composer.end(forceDoc?: boolean, offset?: number): Generator<Document.Parsed>

Always call at end of input to push out any remaining document. If forceDoc is true and the stream contains no document, still emit a final document including any comments and directives that would be applied to a subsequent document. offset should be set if forceDoc is also set, to set the document range end and to indicate errors correctly.

composer.streamInfo(): { comment, directives, errors, warnings }

Current stream status information. Mostly useful at the end of input for an empty stream.

Working with CST Tokens

import { CST } from 'yaml'

For most use cases, the Document or pure JS interfaces provided by the library are the right tool. Sometimes, though, it's important to keep the original YAML source in as pristine a condition as possible. For those cases, the concrete syntax tree (CST) representation is provided, as it retains every character of the input, including whitespace.

CST.createScalarToken(value: string, context): BlockScalar | FlowScalar

Create a new scalar token with the value value. Values that represent an actual string but may be parsed as a different type should use a type other than 'PLAIN', as this function does not support any schema operations and won't check for such conflicts.

Argument Type Default Description
value string The string representation of the value, which will have its content properly indented. Required.
context.end SourceToken[] Comments and whitespace after the end of the value, or after the block scalar header. If undefined, a newline will be added.
context.implicitKey boolean false Being within an implicit key may affect the resolved type of the token's value.
context.indent number The indent level of the token. Required.
context.inFlow boolean false Is this scalar within a flow collection? This may affect the resolved type of the token's value.
context.offset number -1 The offset position of the token.
context.type Scalar.Type The preferred type of the scalar token. If undefined, the previous type of the token will be used, defaulting to 'PLAIN'.
const [doc] = new Parser().parse('foo: "bar" #comment')
const item = doc.value.items[0].value
> {
    type: 'double-quoted-scalar',
    offset: 5,
    indent: 0,
    source: '"bar"',
    end: [
      { type: 'space', offset: 10, indent: 0, source: ' ' },
      { type: 'comment', offset: 11, indent: 0, source: '#comment' }

> { value: 'bar', type: 'QUOTE_DOUBLE', comment: 'comment', range: [5, 9, 19] }

CST.isCollection(token?: Token): boolean

CST.isScalar(token?: Token): boolean

Custom type guards for detecting CST collections and scalars, in both their block and flow forms.

CST.resolveAsScalar(token?: Token, strict = true, onError?: ComposeErrorHandler)

If token is a CST flow or block scalar, determine its string value and a few other attributes. Otherwise, return null.

CST.setScalarValue(token: Token, value: string, context?)

Set the value of token to the given string value, overwriting any previous contents and type that it may have.

Best efforts are made to retain any comments previously associated with the token, though all contents within a collection's items will be overwritten.

Values that represent an actual string but may be parsed as a different type should use a type other than 'PLAIN', as this function does not support any schema operations and won't check for such conflicts.

Argument Type Default Description
token Token Any token. If it does not include an indent value, the value will be stringified as if it were an implicit key. Required.
value string The string representation of the value, which will have its content properly indented. Required.
context.afterKey boolean false In most cases, values after a key should have an additional level of indentation.
context.implicitKey boolean false Being within an implicit key may affect the resolved type of the token's value.
context.inFlow boolean false Being within a flow collection may affect the resolved type of the token's value.
context.type Scalar.Type The preferred type of the scalar token. If undefined, the previous type of the token will be used, defaulting to 'PLAIN'.
function findScalarAtOffset(
  cst: CST.Document,
  offset: number
): CST.FlowScalar | CST.BlockScalar | undefined {
  let res: CST.FlowScalar | CST.BlockScalar | undefined = undefined
  CST.visit(cst, ({ key, value }) => {
    for (const token of [key, value])
      if (CST.isScalar(token)) {
        if (token.offset > offset) return CST.visit.BREAK
        if (
          token.offset == offset ||
          (token.offset < offset && token.offset + token.source.length > offset)
        ) {
          res = token
          return CST.visit.BREAK
  return res

CST.stringify(cst: Token | CollectionItem): string

Stringify a CST document, token, or collection item. Fair warning: This applies no validation whatsoever, and simply concatenates the sources in their logical order.

CST.visit(cst: CST.Document | CST.CollectionItem, visitor: CSTVisitor)

Apply a visitor to a CST document or item. Effectively, the general-purpose workhorse of navigating the CST.

Walks through the tree (depth-first) starting from cst as the root, calling a visitor function with two arguments when entering each item:

The return value of the visitor may be used to control the traversal:

const [doc] = new Parser().parse('[ foo, bar, baz ]')
CST.visit(doc, (item, path) => {
  if (!CST.isScalar(item.value)) return
  const scalar = CST.resolveAsScalar(item.value)
  if (scalar?.value === 'bar') {
    const parent = CST.visit.parentCollection(doc, path)
    const idx = path[path.length - 1][1]
    const { indent } = item.value
    parent.items.splice(idx, 0, {
      start: item.start.slice(),
      value: CST.createScalarToken('bing', { end: [], indent })
    return idx + 2

> '[ foo, bing, bar, baz ]'

A couple of utility functions are provided for working with the path:


Nearly all errors and warnings produced by the yaml parser functions contain the following fields:

Member Type Description
code string An identifier for the error type.
linePos [LinePos, LinePos] ⎮ undefined If prettyErrors is enabled and offset is known, the one-indexed human-friendly source location { line: number, col: number }.
name 'YAMLParseError' ⎮ 'YAMLWarning'
message string A human-readable description of the error
pos [number, number] The position in the source at which this error or warning was encountered.

A YAMLParseError is an error encountered while parsing a source as YAML. They are included in the doc.errors array. If that array is not empty when constructing a native representation of a document, the first error will be thrown.

A YAMLWarning is not an error, but a spec-mandated warning about unsupported directives or a fallback resolution being used for a node with an unavailable tag. They are included in the doc.warnings array.

In rare cases, the library may produce a more generic error. In particular, TypeError may occur when parsing invalid input using the json schema, and ReferenceError when the maxAliasCount limit is enountered.

To identify errors for special handling, you should primarily use code to differentiate them from each other.

Code Description
ALIAS_PROPS Unlike scalars and collections, alias nodes cannot have an anchor or tag associated with it.
BAD_ALIAS An alias identifier must be a non-empty sequence of valid characters.
BAD_DIRECTIVE Only the %YAML and %TAG directives are supported, and they need to follow the specified structure.
BAD_DQ_ESCAPE Double-quotes strings may include \ escaped content, but that needs to be valid.
BAD_INDENT Indentation is important in YAML, and collection items need to all start at the same level. Block scalars are also picky about their leading content.
BAD_PROP_ORDER Anchors and tags must be placed after the ?, : and - indicators.
BAD_SCALAR_START Plain scalars cannot start with a block scalar indicator, or one of the two reserved characters: @ and `. To fix, use a block or quoted scalar for the value.
BLOCK_AS_IMPLICIT_KEY There's probably something wrong with the indentation, or you're trying to parse something like a: b: c, where it's not clear what's the key and what's the value.
BLOCK_IN_FLOW YAML scalars and collections both have block and flow styles. Flow is allowed within block, but not the other way around.
DUPLICATE_KEY Map keys must be unique. Use the uniqueKeys option to disable or customise this check when parsing.
IMPOSSIBLE This really should not happen. If you encounter this error code, please file a bug.
KEY_OVER_1024_CHARS Due to legacy reasons, implicit keys must have their following : indicator after at most 1k characters.
MISSING_ANCHOR Aliases can only dereference anchors that are before them in the document.
MISSING_CHAR Some character or characters are missing here. See the error message for what you need to add.
MULTILINE_IMPLICIT_KEY Implicit keys need to be on a single line. Does the input include a plain scalar with a : followed by whitespace, which is getting parsed as a map key?
MULTIPLE_ANCHORS A node is only allowed to have one anchor.
MULTIPLE_DOCS A YAML stream may include multiple documents. If yours does, you'll need to use parseAllDocuments() to work with it.
MULTIPLE_TAGS A node is only allowed to have one tag.
TAB_AS_INDENT Only spaces are allowed as indentation.
TAG_RESOLVE_FAILED Something went wrong when resolving a node's tag with the current schema.
UNEXPECTED_TOKEN A token was encountered in a place where it wasn't expected.

Silencing Errors and Warnings

Some of the errors encountered during parsing are required by the spec, but are caused by content that may be parsed unambiguously. To ignore these errors, use the strict: false option:

For additional control, set the logLevel option to 'error' (default: 'warn') to silence all warnings. Setting logLevel: 'silent' will ignore parsing errors completely, resulting in output that may well be rather broken.

Command-line Tool

Available as npx yaml or npm exec yaml:

yaml: A command-line YAML processor and inspector

Reads stdin and writes output to stdout and errors & warnings to stderr.

  yaml          Process a YAML stream, outputting it as YAML
  yaml cst      Parse the CST of a YAML stream
  yaml lex      Parse the lexical tokens of a YAML stream
  yaml valid    Validate a YAML stream, returning 0 on success

  --help, -h    Show this message.
  --json, -j    Output JSON.

Additional options for bare "yaml" command:
  --doc, -d     Output pretty-printed JS Document objects.
  --single, -1  Require the input to consist of a single YAML document.
  --strict, -s  Stop on errors.
  --visit, -v   Apply a visitor to each document (requires a path to import)
  --yaml 1.1    Set the YAML version. (default: 1.2)

YAML Syntax

A YAML schema is a combination of a set of tags and a mechanism for resolving non-specific tags, i.e. values that do not have an explicit tag such as !!int. The default schema is the 'core' schema, which is the recommended one for YAML 1.2. For YAML 1.1 documents the default is 'yaml-1.1'.


// '42'

YAML.parse('!!int "42"')
// 42

%TAG !,2018:app/
!foo 42
// YAMLWarning:
//   The tag,2018:app/foo is unavailable,
//   falling back to,2002:str
// '42'

The default prefix for YAML tags is,2002:, for which the shorthand !! is used when stringified. Shorthands for other prefixes may also be defined by document-specific directives, e.g. !e! or just ! for,2018:app/, but this is not required to use a tag with a different prefix.

During parsing, unresolved tags should not result in errors (though they will be noted as warnings), with the tagged value being parsed according to the data type that it would have under automatic tag resolution rules. This should not result in any data loss, allowing such tags to be handled by the calling app.

In order to have yaml provide you with automatic parsing and stringification of non-standard data types, it will need to be configured with a suitable tag object. For more information, see Custom Tags.

The YAML 1.0 tag specification is slightly different from that used in later versions, and implements prefixing shorthands rather differently.

Version Differences

This library's parser is based on the 1.2 version of the YAML spec, which is almost completely backwards-compatible with YAML 1.1 as well as YAML 1.0. Some specific relaxations have been added for backwards compatibility, but if you encounter an issue please report it.

Changes from YAML 1.1 to 1.2

%YAML 1.1
true: Yes
octal: 014
sexagesimal: 3:25:45
picture: !!binary |
{ true: true,
  octal: 12,
  sexagesimal: 12345,
   Buffer [Uint8Array] [
     71, 73, 70, 56, 57, 97, 12, 0, 12, 0, 132, 0, 0,
     255, 255, 247, 245, 245, 238, 233, 233, 229, 102,
     102, 102, 0, 0, 0, 231, 231, 231, 94, 94, 94, 243,
     243, 237, 142, 142, 142, 224, 224, 224, 159, 159,
     159, 147, 147, 147, 167, 167, 167, 158, 158, 158,
     105, 94, 16, 39, 32, 130, 10, 1, 0, 59 ] }

The most significant difference between YAML 1.1 and YAML 1.2 is the introduction of the core data schema as the recommended default, replacing the YAML 1.1 type library:

The other major change has been to make sure that YAML 1.2 is a valid superset of JSON. Additionally there are some minor differences between the parsing rules:

Changes from YAML 1.0 to 1.1

date: 2001-01-23
number: !int '123'
string: !str 123
pool: !!ball { number: 8 }
invoice: !domain.tld,2002/^invoice
  customers: !seq
    - !^customer
      given : Chris
      family : Dumars

The most significant difference between these versions is the complete refactoring of the tag syntax:

Additionally, the formal description of the language describing the document structure has been completely refactored between these versions, but the described intent has not changed. Other changes include:

yaml@1 supports parsing and stringifying YAML 1.0 documents, but does not expand tags using the ^ notation. As there is no indication that anyone is still using YAML 1.0, explicit support has been dropped in yaml@2.