Master YAML: Essential Syntax, Data Types, and Advanced Features Explained

YAML is a concise and powerful language designed for writing configuration files.

YAML stands for "YAML Ain’t a Markup Language" and its design draws from JSON, XML, and SDL, emphasizing data‑centric, readable, and easy‑to‑write syntax.

YAML Syntax Features

People with programming experience find it very easy to understand.

Syntax Characteristics

Case‑sensitive

Uses indentation to represent hierarchy

Tab indentation is prohibited; only spaces are allowed

The number of spaces for indentation is not important as long as it is consistent within the same level

Comments are indicated with

#

# yaml
languages:
  - Ruby
  - Perl
  - Python
websites:
  YAML: yaml.org
  Ruby: ruby-lang.org
  Python: python.org
  Perl: use.perl.org

# Json
{
  "languages": [
    "Ruby",
    "Perl",
    "Python"
  ],
  "websites": {
    "YAML": "yaml.org",
    "Ruby": "ruby-lang.org",
    "Python": "python.org",
    "Perl": "use.perl.org"
  }
}

Data Structures

Object: a dictionary of key‑value pairs

Array: an ordered list of items

Scalar: a single indivisible value

# scalar
hello

# array
- Cat
- Dog
- Goldfish

# object
animal: pets

Quote Differences

Single quotes ( ''): treat special characters as literal strings

Double quotes ( ""): interpret special characters

# single quotes
name: 'Hi,
Tom'

# double quotes
name: "Hi,
Tom"

Built‑in Types List

!!int

: integer type !!float: floating‑point type !!bool: boolean type !!str: string type !!null: null value !!set: set collection !!seq: list (sequence) !!map: mapping (dictionary) !!binary: binary data !!timestamp: date‑time type !!omap / !!pairs: ordered map / key‑value list

YAML Scalars

Scalars are the most basic and indivisible values.

String

# without quotes
name: Tom

# using single quotes
name: 'Tom'

# using double quotes
name: "Tom"

Boolean

debug: true
debug: false

Number

# decimal integer
12       # ten‑base integer
014      # octal integer
0xC      # hexadecimal integer
13.4     # floating‑point
1.2e+34  # exponent
.inf     # infinity

Null

# various null representations
date: ~
date: null

Timestamp

# ISO‑8601 format
canonical: 2001-12-15T02:59:43.1Z
valid iso8601: 2001-12-14t21:59:43.10-05:00
space separated: 2001-12-14 21:59:43.10 -5
no time zone (Z): 2001-12-15 2:59:43.10
date only (00:00:00Z): 2002-12-14

JavaScript Specific Types

# regular expression
simple: !!js/regexp foobar
modifiers: !!js/regexp /foobar/mi

# undefined
undefined: !!js/undefined ~

# function
function: !!js/function >
  function foobar() {
    return 'Wow! JS-YAML Rocks!';
  }

Custom Types

JS‑YAML allows defining custom YAML types for your structures. Example of a custom !sexy type:

foobar: !sexy
  - bunny
  - chocolate