JSON Cheat Sheet

A quick reference for JSON syntax, data types, and common patterns.

Data Types

Type	Example	Notes
String	`"hello world"`	Must use double quotes
Number	`42`, `3.14`, `-1`, `1.5e10`	No leading zeros, no hex/octal
Boolean	`true`, `false`	Lowercase only
Null	`null`	Lowercase only
Object	`{"key": "value"}`	Keys must be double-quoted strings
Array	`[1, 2, 3]`	Ordered list of values

Basic Structure

{
  "name": "Alice",
  "age": 30,
  "active": true,
  "address": {
    "city": "New York",
    "zip": "10001"
  },
  "tags": ["developer", "designer"],
  "notes": null
}

String Escaping

Escape	Character
`\"`	Double quote
`\\`	Backslash
`\n`	Newline
`\t`	Tab
`\r`	Carriage return
`\uXXXX`	Unicode character

Common Mistakes

Trailing comma

{ "a": 1, "b": 2, }  // ← Invalid!

Single quotes

{ 'name': 'Alice' }  // ← Invalid!

Unquoted keys

{ name: "Alice" }  // ← Invalid!

Comments

{
  // This is not allowed in JSON
  "key": "value"
}

Undefined values

{ "key": undefined }  // ← Invalid! Use null

JSON vs JavaScript

Feature	JSON	JavaScript
Quotes for keys	Required (double)	Optional
String quotes	Double only	Single, double, or backtick
Trailing commas	Not allowed	Allowed
Comments	Not allowed	Allowed
undefined	Not allowed	Valid value