JS-YAML - YAML 1.2 parser / writer for JavaScript

Online Demo

A fast and complete YAML parser and writer for JavaScript. Supports both the 1.2 and 1.1 specs, and passes the entire YAML Test Suite.

Installation

npm install js-yaml

Upgrading from v4? See the v5 migration guide.

API

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

import { load } from 'js-yaml'
import { readFileSync } from 'node:fs'

// Get document, or throw exception on error
try {
  const doc = load(readFileSync('example.yml', 'utf8'))
  console.log(doc)
} catch (e) {
  console.log(e)
}

load (string [ , options ])

Parses string as a single YAML document. Throws YAMLException on error. This function does not understand multi-document or empty sources; it throws an exception on those.

[!WARNING] When processing untrusted input, see the security considerations.

options:

• filename (default: null) - string to be used as a file path in error/warning messages.
• schema (default: CORE_SCHEMA) - specifies a schema to use.
  • FAILSAFE_SCHEMA - only strings, arrays and plain objects.
  • JSON_SCHEMA - all JSON-supported types.
  • CORE_SCHEMA - a superset of JSON_SCHEMA, accepting more notations for the same types.
  • YAML11_SCHEMA - adds the legacy YAML 1.1 types (!!binary, !!timestamp, !!omap, !!pairs, !!set, merge keys <<, and the broader 1.1 scalar notations).
• json (default: false) - compatibility with JSON.parse behaviour. If true, duplicate keys in a mapping override values rather than throwing an error.
• maxDepth (default: 100) - limits the nesting depth for collections (does not take aliases into account).
• maxTotalMergeKeys (default: 10000) - limits the total number of keys processed by merge (<<) across one load() / loadAll() call. Set to -1 to disable.
• maxAliases (default: -1) - limits the number of alias nodes (*ref) per document. Set to 0 to reject all aliases, or to -1 for no limit.

[!NOTE]

The default CORE_SCHEMA comes without the !!merge tag. You can easily enable it if needed:

import { load, CORE_SCHEMA, mergeTag } from 'js-yaml'

load(data, { schema: CORE_SCHEMA.withTags(mergeTag) })

[!WARNING]

The default mapTag is {}-object based and does not allow complex keys (objects, arrays and so on). That's an intentional choice for convenience. Also, non-string scalar keys, such as null, numbers or booleans, are converted to strings.

In the rare cases where you really need complex keys, use realMapTag in the schema instead. It stores any key exactly as provided, at the cost of less convenient access.

See examples for advanced customization approaches.

loadAll (string [, options ])

Same as load(), but understands multi-document sources. Returns an array of documents.

import { loadAll } from 'js-yaml'

console.log(loadAll(data))

dump (object [ , options ])

Serializes object as a YAML document. By default it can dump every supported YAML type, so it throws an exception if you try to dump regexps or functions. However, you can disable exceptions by setting the skipInvalid option to true.

options:

• indent (default: 2) - indentation width to use (in spaces).
• flowLevel (default: -1) - nesting level at which collections switch from block to flow style (-1 means never).
• seqNoIndent (default: false) - when true, does not add an indentation level to array elements, ␣␣- 1 => - 1.
• seqInlineFirst (default: true) - when true, allows a nested collection to start on the same line after -, -\n - 1 => - - 1.
• skipInvalid (default: false) - do not throw on invalid types (such as a function in the schema). Invalid mapping pairs and sequence items are skipped; undefined sequence items are serialized as null.
• schema (default: a YAML11_SCHEMA-based 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) - sets the max line width. Set -1 for unlimited width.
• noRefs (default: false) - if true, don't convert duplicate objects into references; inline them instead.
• quoteStyle (single or double, default: single) - quoting style to use when a string needs quotes.
• forceQuotes (default: false) - if true, quote all non-key strings, using quoteStyle.
• flowBracketPadding (default: false) - add spaces inside flow collection brackets, {a: 1} => { a: 1 }.
• flowSkipCommaSpace (default: false) - omit the space after commas in flow collections, [1, 2] => [1,2].
• flowSkipColonSpace (default: false) - omit the space after : in flow mappings, {a: 1} => {a:1}.
• quoteFlowKeys (default: false) - quote flow mapping keys, {a: 1} => {"a": 1}.
• tagBeforeAnchor (default: false) - print an explicit tag before an anchor, &ref_0 !!set => !!set &ref_0.
• transform - a function (documents: Document[]) => void that can mutate the generated AST before it is rendered.

See examples for advanced customization approaches.

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 'true'               # bool
!!int '3...'                # number
!!float '3.14...'           # number
!!str '...'                 # string
!!seq [ ... ]               # array
!!map { ... }               # object (or Map)

The types below are only available in YAML11_SCHEMA (not in the default CORE_SCHEMA):

!!binary '...base64...'     # Uint8Array
!!timestamp 'YYYY-...'      # date
!!set { ... }               # Set

# Legacy YAML 1.1 compatibility only; these types cannot be dumped.
!!omap [ ... ]              # array of key-value pairs
!!pairs [ ... ]             # array of array pairs

To preserve complex keys in the first position of a !!pairs item, replace the default object-based map with realMapTag in the schema.

JavaScript-specific tags

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

CLI

This can be useful sometimes for a quick check.

npx js-yaml -h

Note: the CLI script comes with minimal options, and there are no big plans to extend it.