js-yaml

What is js-yaml?

The js-yaml npm package is a JavaScript library that allows you to parse and dump YAML, a human-friendly data serialization standard. It can be used to convert YAML to JSON and vice versa, making it a useful tool for configuration files, data exchange, and more.

What are js-yaml's main functionalities?

YAML Parsing

This feature allows you to parse a YAML file or string and convert it into a JavaScript object. The code sample demonstrates how to read a YAML file from the filesystem and parse its contents.

{"const yaml = require('js-yaml');\nconst fs = require('fs');\n\ntry {\n  const doc = yaml.load(fs.readFileSync('/path/to/file.yml', 'utf8'));\n  console.log(doc);\n} catch (e) {\n  console.error(e);\n}"}

YAML Dumping

This feature allows you to take a JavaScript object and convert it into a YAML-formatted string. The code sample shows how to create a YAML string from an object and then save it to a file.

{"const yaml = require('js-yaml');\nconst fs = require('fs');\n\nconst obj = { hello: 'world' };\nconst ymlText = yaml.dump(obj);\n\nfs.writeFileSync('/path/to/file.yml', ymlText, 'utf8');\nconsole.log('YAML file saved.');"}

Custom Types

js-yaml allows you to define custom types for specialized use cases. The code sample demonstrates how to create a custom YAML type and use it in parsing a YAML string.

{"const yaml = require('js-yaml');\n\nconst schema = yaml.Schema.create([ new yaml.Type('!myType', {\n  kind: 'scalar',\n  resolve: data => data === 'valid',\n  construct: data => data,\n  instanceOf: String\n}) ]);\n\nconst doc = yaml.load('!myType valid', { schema });\nconsole.log(doc); // 'valid'"}

Other packages similar to js-yaml

yaml

The 'yaml' package is another JavaScript library for parsing and serializing YAML. It offers a similar API to js-yaml but with a focus on being highly compliant with the YAML specification. It may be preferred for applications that require strict adherence to the spec.

yamljs

Yamljs is a JavaScript library that provides YAML parsing and dumping functionalities. It is similar to js-yaml but has a different API design and may not be as actively maintained as js-yaml.

yaml-ast-parser

This package is designed for parsing YAML into an abstract syntax tree (AST). It is useful for developers who need to analyze or manipulate the structure of YAML documents at a lower level compared to js-yaml.

README

JS-YAML - YAML 1.2 parser / writer for JavaScript

Online Demo

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.

Installation

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

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

Bundled YAML library for browsers

<!-- esprima required only for !!js/function -->
<script src="esprima.js"></script>
<script src="js-yaml.min.js"></script>
<script type="text/javascript">
var doc = jsyaml.load('greeting: hello\nname: world');
</script>

Browser support was done mostly for the online demo. If you find any errors - feel free to send pull requests with fixes. Also note, that IE and other old browsers needs es5-shims to operate.

Notes:

1. We have no resources to support browserified version. Don't expect it to be well tested. Don't expect fast fixes if something goes wrong there.
2. !!js/function in browser bundle will not work by default. If you really need it - load esprima parser first (via amd or directly).
3. !!bin in browser will return Array, because browsers do not support node.js Buffer and adding Buffer shims is completely useless on practice.

API

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

const yaml = require('js-yaml');
const fs   = require('fs');

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

safeLoad (string [ , options ])

Recommended loading way. Parses string as single YAML document. Returns either a plain object, a string or undefined, or throws YAMLException on error. By default, does not support regexps, functions and undefined. This method is safe for untrusted data.

options:

• 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_SAFE_SCHEMA) - specifies a schema to use.
  • FAILSAFE_SCHEMA - only strings, arrays and plain objects: http://www.yaml.org/spec/1.2/spec.html#id2802346
  • JSON_SCHEMA - all JSON-supported types: http://www.yaml.org/spec/1.2/spec.html#id2803231
  • CORE_SCHEMA - same as JSON_SCHEMA: http://www.yaml.org/spec/1.2/spec.html#id2804923
  • DEFAULT_SAFE_SCHEMA - all supported YAML types, without unsafe ones (!!js/undefined, !!js/regexp and !!js/function): http://yaml.org/type/
  • DEFAULT_FULL_SCHEMA - all supported YAML types.
• 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.

load (string [ , options ])

Use with care with untrusted sources. The same as safeLoad() but uses DEFAULT_FULL_SCHEMA by default - adds some JavaScript-specific types: !!js/function, !!js/regexp and !!js/undefined. For untrusted sources, you must additionally validate object structure to avoid injections:

const untrusted_code = '"toString": !<tag:yaml.org,2002:js/function> "function (){very_evil_thing();}"';

// I'm just converting that string, what could possibly go wrong?
require('js-yaml').load(untrusted_code) + ''

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

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

const yaml = require('js-yaml');

yaml.safeLoadAll(data, function (doc) {
  console.log(doc);
});

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

Same as safeLoadAll() but uses DEFAULT_FULL_SCHEMA by default.

safeDump (object [ , options ])

Serializes object as a YAML document. Uses DEFAULT_SAFE_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.

options:

• 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_SAFE_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.
• 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.

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"       -> "01", "052", "016172"
  "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'

Example:

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

dump (object [ , options ])

Same as safeDump() but without limits (uses DEFAULT_FULL_SCHEMA by default).

Supported YAML types

The list of standard YAML tags and corresponding JavaScipt 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

!!js/regexp /pattern/gim            # RegExp
!!js/undefined ''                   # Undefined
!!js/function 'function () {...}'   # Function

Caveats

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

js-yaml for enterprise

Available as part of the Tidelift Subscription

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Changelog

[3.14.1] - 2020-12-07

Security

• Fix possible code execution in (already unsafe) .load() (in &anchor).