JSON Syntax Cheat Sheet

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JSON data types, syntax rules, nested structures, and common patterns

Data Types

String

Double-quoted Unicode text. Escape special characters.

"Hello, World!"
"Line 1\nLine 2"
"Tab\there"
"Quote: \"word\""
"Unicode: \u0041"

Number

Integer or floating-point. No hex, no leading zeros.

42
-17
3.14
1.5e10
-2.5e-3

// Not valid JSON:
// 0xFF (hex)
// 01 (leading zero)

Boolean & Null

Lowercase true, false, and null only

true
false
null

// Not valid: True, False, None, undefined

Array

Ordered list of values in square brackets

[1, 2, 3]
["a", "b", "c"]
[true, null, 42, "mixed"]
[{"id": 1}, {"id": 2}]

Object

Unordered collection of key-value pairs in curly braces

{
  "name": "Alice",
  "age": 30,
  "active": true
}

Syntax Rules

Strings Must Use Double Quotes

Single quotes are not valid JSON

// Valid
{ "name": "Alice" }

// Invalid
{ 'name': 'Alice' }  // single quotes!

No Trailing Commas

JSON does not allow trailing commas after the last item

// Valid
{ "a": 1, "b": 2 }

// Invalid
{ "a": 1, "b": 2, }  // trailing comma!

No Comments

JSON does not support // or /* */ comments

// Invalid JSON:
{
  // "comment": "this is not allowed",
  "name": "Alice"
}

// Workaround: use a "_comment" key
{ "_comment": "describes the object", "name": "Alice" }

Keys Must Be Strings

Object keys must always be double-quoted strings

// Valid
{ "key": "value" }

// Invalid
{ key: "value" }     // unquoted key
{ 42: "value" }      // numeric key

JSON Methods in JS

JSON.parse()

Parse a JSON string into a JavaScript value

const data = JSON.parse('{"name":"Alice","age":30}');
console.log(data.name); // "Alice"

// With reviver function
const parsed = JSON.parse(text, (key, value) => {
  if (key === "date") return new Date(value);
  return value;
});

JSON.stringify()

Convert a JavaScript value to a JSON string

JSON.stringify({ a: 1 })        // '{"a":1}'
JSON.stringify({ a: 1 }, null, 2)  // pretty-print

// With replacer (filter/transform keys)
JSON.stringify(obj, ["name", "age"])  // only include these keys
JSON.stringify(obj, (k, v) => typeof v === "function" ? undefined : v)

JSON vs JavaScript

Key Differences

JSON is stricter than JavaScript object literals

// JavaScript allows:        // JSON requires:
key: "value"                  "key": "value"  (quoted)
'single quotes'               "double quotes"
{ a: 1, }                     { "a": 1 }      (no trailing comma)
undefined                     null            (undefined not valid)
// comments OK                // no comments!
function() {}                 not allowed     (functions not valid)

Common Patterns

API Response Object

Standard REST API response envelope

{
  "success": true,
  "data": {
    "id": 42,
    "name": "Alice Johnson",
    "email": "alice@example.com",
    "createdAt": "2024-01-15T10:30:00Z",
    "roles": ["admin", "editor"],
    "profile": {
      "avatar": "https://example.com/alice.jpg",
      "bio": null
    }
  },
  "meta": {
    "requestId": "abc-123",
    "timestamp": "2024-01-15T10:30:01Z"
  }
}

Configuration File

JSON configuration pattern for apps and tools

{
  "name": "my-app",
  "version": "1.0.0",
  "settings": {
    "theme": "dark",
    "language": "en",
    "notifications": {
      "email": true,
      "push": false
    }
  },
  "database": {
    "host": "localhost",
    "port": 5432,
    "name": "mydb"
  },
  "features": {
    "beta": false,
    "darkMode": true
  }
}

Array of Objects

Common JSON structure for lists and collections

{
  "users": [
    {
      "id": 1,
      "name": "Alice",
      "email": "alice@example.com",
      "active": true
    },
    {
      "id": 2,
      "name": "Bob",
      "email": "bob@example.com",
      "active": false
    }
  ],
  "pagination": {
    "total": 2,
    "page": 1,
    "limit": 20
  }
}