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YAML 1.2 parser and serializer


Version published
Weekly downloads
decreased by-11.51%

Weekly downloads



[4.1.0] - 2021-04-15


  • Types are now exported as yaml.types.XXX.
  • Every type now has options property with original arguments kept as they were (see as an example).


  • Schema.extend() now keeps old type order in case of conflicts (e.g. Schema.extend([ a, b, c ]).extend([ b, a, d ]) is now ordered as abcd instead of cbad).



JS-YAML - YAML 1.2 parser / writer for JavaScript

CI NPM version

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This is an implementation of YAML, a human-friendly data serialization language. Started as PyYAML port, it was completely rewritten from scratch. Now it's very fast, and supports 1.2 spec.


YAML module for node.js

npm install js-yaml

CLI executable

If you want to inspect your YAML files from CLI, install js-yaml globally:

npm install -g js-yaml
usage: js-yaml [-h] [-v] [-c] [-t] file Positional arguments: file File with YAML document(s) Optional arguments: -h, --help Show this help message and exit. -v, --version Show program's version number and exit. -c, --compact Display errors in compact mode -t, --trace Show stack trace on error


Here we cover the most 'useful' methods. If you need advanced details (creating your own tags), see examples for more info.

const yaml = require('js-yaml'); const fs = require('fs'); // Get document, or throw exception on error try { const doc = yaml.load(fs.readFileSync('/home/ixti/example.yml', 'utf8')); console.log(doc); } catch (e) { console.log(e); }

load (string [ , options ])

Parses string as single YAML document. Returns either a plain object, a string, a number, null or undefined, or throws YAMLException on error. By default, does not support regexps, functions and undefined.


  • filename (default: null) - string to be used as a file path in error/warning messages.
  • onWarning (default: null) - function to call on warning messages. Loader will call this function with an instance of YAMLException for each warning.
  • schema (default: DEFAULT_SCHEMA) - specifies a schema to use.
  • json (default: false) - compatibility with JSON.parse behaviour. If true, then duplicate keys in a mapping will override values rather than throwing an error.

NOTE: This function does not understand multi-document sources, it throws exception on those.

NOTE: JS-YAML does not support schema-specific tag resolution restrictions. So, the JSON schema is not as strictly defined in the YAML specification. It allows numbers in any notation, use Null and NULL as null, etc. The core schema also has no such restrictions. It allows binary notation for integers.

loadAll (string [, iterator] [, options ])

Same as load(), but understands multi-document sources. Applies iterator to each document if specified, or returns array of documents.

const yaml = require('js-yaml'); yaml.loadAll(data, function (doc) { console.log(doc); });

dump (object [ , options ])

Serializes object as a YAML document. Uses DEFAULT_SCHEMA, so it will throw an exception if you try to dump regexps or functions. However, you can disable exceptions by setting the skipInvalid option to true.


  • indent (default: 2) - indentation width to use (in spaces).
  • noArrayIndent (default: false) - when true, will not add an indentation level to array elements
  • skipInvalid (default: false) - do not throw on invalid types (like function in the safe schema) and skip pairs and single values with such types.
  • flowLevel (default: -1) - specifies level of nesting, when to switch from block to flow style for collections. -1 means block style everwhere
  • styles - "tag" => "style" map. Each tag may have own set of styles.
  • schema (default: DEFAULT_SCHEMA) specifies a schema to use.
  • sortKeys (default: false) - if true, sort keys when dumping YAML. If a function, use the function to sort the keys.
  • lineWidth (default: 80) - set max line width. Set -1 for unlimited width.
  • noRefs (default: false) - if true, don't convert duplicate objects into references
  • noCompatMode (default: false) - if true don't try to be compatible with older yaml versions. Currently: don't quote "yes", "no" and so on, as required for YAML 1.1
  • condenseFlow (default: false) - if true flow sequences will be condensed, omitting the space between a, b. Eg. '[a,b]', and omitting the space between key: value and quoting the key. Eg. '{"a":b}' Can be useful when using yaml for pretty URL query params as spaces are %-encoded.
  • quotingType (' or ", default: ') - strings will be quoted using this quoting style. If you specify single quotes, double quotes will still be used for non-printable characters.
  • forceQuotes (default: false) - if true, all non-key strings will be quoted even if they normally don't need to.
  • replacer - callback function (key, value) called recursively on each key/value in source object (see replacer docs for JSON.stringify).

The following table show availlable styles (e.g. "canonical", "binary"...) available for each tag (.e.g. !!null, !!int ...). Yaml output is shown on the right side after => (default setting) or ->:

!!null "canonical" -> "~" "lowercase" => "null" "uppercase" -> "NULL" "camelcase" -> "Null" !!int "binary" -> "0b1", "0b101010", "0b1110001111010" "octal" -> "0o1", "0o52", "0o16172" "decimal" => "1", "42", "7290" "hexadecimal" -> "0x1", "0x2A", "0x1C7A" !!bool "lowercase" => "true", "false" "uppercase" -> "TRUE", "FALSE" "camelcase" -> "True", "False" !!float "lowercase" => ".nan", '.inf' "uppercase" -> ".NAN", '.INF' "camelcase" -> ".NaN", '.Inf'


dump(object, { 'styles': { '!!null': 'canonical' // dump null as ~ }, 'sortKeys': true // sort object keys });

Supported YAML types

The list of standard YAML tags and corresponding JavaScript types. See also YAML tag discussion and YAML types repository.

!!null '' # null !!bool 'yes' # bool !!int '3...' # number !!float '3.14...' # number !!binary '...base64...' # buffer !!timestamp 'YYYY-...' # date !!omap [ ... ] # array of key-value pairs !!pairs [ ... ] # array or array pairs !!set { ... } # array of objects with given keys and null values !!str '...' # string !!seq [ ... ] # array !!map { ... } # object

JavaScript-specific tags

See js-yaml-js-types for extra types.


Note, that you use arrays or objects as key in JS-YAML. JS does not allow objects or arrays as keys, and stringifies (by calling toString() method) them at the moment of adding them.

--- ? [ foo, bar ] : - baz ? { foo: bar } : - baz - baz { "foo,bar": ["baz"], "[object Object]": ["baz", "baz"] }

Also, reading of properties on implicit block mapping keys is not supported yet. So, the following YAML document cannot be loaded.

&anchor foo: foo: bar *anchor: duplicate key baz: bat *anchor: duplicate key

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What is js-yaml?

YAML 1.2 parser and serializer

Is js-yaml popular?

The npm package js-yaml receives a total of 42,216,272 weekly downloads. As such, js-yaml popularity was classified as popular.

Is js-yaml well maintained?

We found that js-yaml demonstrated a not healthy version release cadence and project activity because the last version was released a year ago.It has 1 open source maintainer collaborating on the project.

Last updated on 14 Apr 2021

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