JSON Basics: Syntax, Common Errors, and How to Fix Them

JSON (JavaScript Object Notation) is the most widely used format for exchanging data between servers, APIs, and applications. It's readable, lightweight, and supported natively by nearly every programming language. Here's what you need to know to write it correctly.

The building blocks

JSON has exactly six data types: strings, numbers, booleans (true/false), null, objects, and arrays. An object is a set of key-value pairs wrapped in curly braces; an array is an ordered list wrapped in square brackets.

{
  "name": "Ada Lovelace",
  "born": 1815,
  "isProgrammer": true,
  "languages": ["Analytical Engine notation"],
  "employer": null
}

The five mistakes that break JSON

Mistake	Wrong	Right
Trailing comma	{"a":1,}	{"a":1}
Unquoted keys	{a:1}	{"a":1}
Single quotes	{'a':'b'}	{"a":"b"}
Comments	{"a":1 // note}	Not allowed in JSON at all
Undefined / functions	{"a":undefined}	Use null instead

Why validators give confusing error messages

Most JSON parsers stop at the exact character where the structure stops making sense — which is often after the actual mistake. If you get an error at line 4, check line 3 first; a missing comma or unclosed brace on an earlier line is the usual culprit.

Try it yourself

Paste your JSON into the JSON Formatter & Validator to get an instant, precise error location and a cleanly indented result.