JSON Syntax: Quick Reference Guide

This post gathers JSON’s syntax rules in one place. A quick reference to check whenever a question comes up, without needing to reread longer posts about each specific topic.

Overall structure

A JSON document is always a single value at the top, usually an object or array (though technically an isolated number, string, boolean, or null is also valid JSON).

{"name": "Ana"}
["item1", "item2"]

Keys (in objects)

  • Always in double quotes
  • Never single quotes, never unquoted
{"name": "Ana"}

Strings

  • Always in double quotes
  • Special characters need to be escaped: \", \\, \/, \n, \t, \r
"line 1\nline 2"

Numbers

  • No quotes
  • No leading zero (01 is invalid)
  • Decimal point needs a digit before and after (.5 and 5. are invalid)
  • Scientific notation is accepted (1.5e10)
42
-7
3.14
1.5e10

Booleans

  • Only true or false, no quotes, always lowercase
true

Null

  • Only null, no quotes, always lowercase
null

Commas

  • Separate items inside objects and arrays
  • There should never be a comma after the last item (trailing comma)
{"a": 1, "b": 2}

What is NOT allowed in JSON

  • Comments (// or /* */)
  • Trailing comma
  • Single quotes
  • Unquoted keys
  • undefined, NaN, Infinity
  • Functions

Nesting

Objects and arrays can be nested freely, in any combination and at any depth.

{
  "name": "Ana",
  "addresses": [
    { "type": "home", "city": "New York" },
    { "type": "work", "city": "Boston" }
  ]
}

Whitespace

Spaces, tabs, and line breaks between elements are ignored by the parser and only serve for readable formatting. These two examples are equivalent:

{"name":"Ana","age":30}
{
  "name": "Ana",
  "age": 30
}

Encoding

JSON should be encoded in UTF-8 by default (the spec also allows UTF-16 and UTF-32, but UTF-8 is by far the most common in practice).

Quick syntax checklist

  • Keys and strings in double quotes?
  • No trailing comma at the end of objects/arrays?
  • No comments?
  • Numbers with no leading zero and with digits before/after the decimal point?
  • true, false, null always lowercase, no quotes?

To quickly validate whether a JSON follows all these rules, paste it into JSON Validator.

{»}
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