JSON Syntax

What is JSON?

JSON is a text-based format built from key–value pairs and ordered lists. Servers send it in HTTP bodies; browsers and runtimes turn it into native types. The formal rules live in RFC 8259; this page is a practical tour of the surface syntax.

Why JSON works well

JSON syntax rules

Read this checklist once; when something fails validation, one of these rules is usually the culprit.

1. Six value types — Every value is a string, number, object, array, boolean, or null. See the data types page for details and edge cases (for example strict number rules).
   • String — Unicode text in double quotes, with escapes as needed.
   • Number — Decimal digits; optional -, fraction, and exponent.
   • Boolean — Literals true or false (never quoted).
   • Array — [ ] around a comma-separated list of values.
   • Object — { } around comma-separated "key": value pairs.
   • Null — Literal null (lowercase).
2. Key–value pairs — Inside objects, each key is a JSON string, followed by :, then the value. Members are separated by , with no trailing comma after the last one.
3. Arrays — Values in [ ] are separated by commas; elements may mix types.
4. Objects — Unordered maps in { }; keys must be unique within one object.

Example of JSON syntax

One object describes a person: primitives for simple fields, an array for hobbies, a nested object for address, and null for an unknown boolean state.

{
  "name": "John Doe",
  "age": 30,
  "isStudent": false,
  "hobbies": ["reading", "traveling", "photography"],
  "address": {
    "street": "123 Main St",
    "city": "Anytown",
    "country": "USA"
  },
  "isMarried": null
}

How to read it

Parsing JSON in JavaScript

Use JSON.parse to turn JSON text into a plain object you can read with dot or bracket notation.

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

For malformed input, JSON.parse throws SyntaxError. The JSON.parse() tutorial covers errors, the reviver callback, and safe handling. To serialize back to text, use JSON.stringify().