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Guide
JSON Basics
Learn the universal data format used in every web API.
Practice with the tool:
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What is JSON?
JSON (JavaScript Object Notation) is a lightweight text format for storing and exchanging structured data. It is the dominant format for web APIs and configuration files.
{
"name": "Alice",
"age": 30,
"active": true,
"tags": ["developer", "designer"]
}
Data Types
JSON supports exactly 6 value types — nothing more, nothing less.
| Type | Example | Notes |
|---|---|---|
| String | "hello" |
Always double-quoted |
| Number | 42, 3.14 |
No quotes, no NaN/Infinity |
| Boolean | true, false |
Lowercase only |
| Null | null |
Represents absence of value |
| Array | [1, "a", true] |
Ordered, mixed types allowed |
| Object | {"key": "val"} |
Unordered key-value pairs |
Syntax Rules
- Keys must be double-quoted strings
- No trailing commas after the last property
- No comments — JSON has no comment syntax
- Strings use double quotes only — not single quotes
{ 'name': 'Alice', age: 30, }
❌ Invalid — single quotes, unquoted key, trailing comma.
{ "name": "Alice", "age": 30 }
✅ Valid.
Common Mistakes
Trailing comma — the most common parsing error:
{ "name": "Alice", } // ❌
{ "name": "Alice" } // ✅
Undefined / NaN — these don't exist in JSON. Use null instead:
{ "value": undefined } // ❌
{ "value": null } // ✅
Numeric string vs number — "42" and 42 are different types. Only use numbers for arithmetic.