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What Is JSON? A Complete Guide

JSON (JavaScript Object Notation) is a lightweight data format for storing and exchanging data. Here's everything you need to know.

The Basics

Full name

JavaScript Object Notation

Pronunciation

"JAY-son" (like the name Jason)

Created by

Douglas Crockford, early 2000s

Standard

RFC 8259

Language support

Python, Java, Go, Ruby, C#, and every major language

File extension

.json

JSON Syntax Rules

  1. 1

    Data is in key/value pairs — keys must be double-quoted strings

  2. 2

    Data is separated by commas

  3. 3

    Objects use curly braces {}

  4. 4

    Arrays use square brackets []

  5. 5

    No trailing commas allowed

  6. 6

    No comments allowed (use "_comment" key as workaround)

Valid example

{
  "name": "Alice",
  "age": 30,
  "active": true,
  "score": 9.5,
  "address": null,
  "tags": ["dev", "admin"]
}

JSON Data Types

String

"hello"

Always double quotes

Number

42 / 3.14

Integer or float, no quotes

Boolean

true / false

Lowercase only

Null

null

Represents empty/unknown

Array

[1, "a", true]

Ordered list, mixed types OK

Object

{"key": "val"}

Key-value pairs, keys must be strings

JSON vs Other Formats

JSON vs CSV

JSON supports nested data and types; CSV is simpler for tabular data.

Full comparison

JSON vs XML

JSON is more compact; XML supports attributes and comments.

Full comparison

JSON vs YAML

YAML is more human-friendly for config; JSON is better for APIs.

Common JSON Mistakes

DON'T

Use single quotes — {'name': 'Alice'} ❌

DON'T

Leave trailing commas — {"a": 1,} ❌

DON'T

Add comments — // this fails ❌

DON'T

Use undefined — not a valid JSON value ❌

DO

Use double quotes for all keys and strings ✓

DO

Validate with a JSON formatter before using ✓

JSON Special Values

Booleans

true and false — lowercase only. True, False, and TRUE are all invalid JSON.

Null

null represents an absent or unknown value. It is distinct from 0, false, and "" — each has a different semantic meaning.

Dates

JSON has no native date type. Common approaches: ISO 8601 string, Unix timestamp, or a custom format. Always agree on a format with your API.

ISO 8601 (recommended)

"2024-01-15T10:30:00Z"

Unix timestamp

1705312200

Custom format

"2024-01-15"

Escape Characters

Special characters inside JSON strings must be escaped with a backslash.

SequenceMeaning
\"double quote
\\backslash
\nnewline
\ttab
\/forward slash (optional)

Work with JSON — Free Online Tools

JSON FormatterFormat, validate, and minify JSON
JSON to CSVExport JSON arrays as CSV files
CSV to JSONParse CSV into JSON objects
JSON to ExcelDownload as XLSX workbook
JSON CompareDiff two JSON objects side by side
JSON to TypeScriptGenerate TypeScript interfaces
JSON EscapeEscape special characters in JSON strings
JSON UnescapeRemove backslash escapes from JSON strings

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