JSON Syntax

JSON syntax is a small, strict set of rules for writing name/value pairs, objects, and arrays as text.

The Core Building Block: Name/Value Pairs

Every piece of data in a JSON object is written as a name/value pair: a name (also called a key), a colon, and a value. The name must be a string wrapped in double quotes, and multiple pairs inside an object are separated by commas.

Name/Value Pairs

<!DOCTYPE html>
<html>
<body>

<h2>Result</h2>
<p id="demo"></p>

<script>
const data = {
  "firstName": "Grace",
  "lastName": "Hopper",
  "yearsActive": 43
};

document.getElementById("demo").innerHTML = data.firstName + " " + data.lastName;
</script>

</body>
</html>

Quoting Rules

JSON is strict about quoting. Keys must always use double quotes — single quotes are not valid JSON, even though they are valid in JavaScript object literals. String values must also use double quotes. Numbers, booleans, and null are written without any quotes at all.

  • Keys: always double-quoted, e.g. "age": 30
  • String values: always double-quoted, e.g. "status": "active"
  • Numbers: no quotes, e.g. "age": 30
  • Booleans: no quotes, lowercase only — true or false
  • Null: no quotes, lowercase only — null
Note: Single-quoted strings like {'name': 'Sam'} are NOT valid JSON. Most parsers will throw a SyntaxError. Always use double quotes.

Objects and Arrays

An object is an unordered set of name/value pairs wrapped in curly braces { }. An array is an ordered list of values wrapped in square brackets [ ]. Arrays do not require names for each entry — just the values themselves, separated by commas.

An Object Containing an Array

<!DOCTYPE html>
<html>
<body>

<h2>Result</h2>
<p id="demo"></p>

<script>
const data = {
  "student": "Miguel",
  "scores": [88, 92, 79, 95]
};

document.getElementById("demo").innerHTML = data.student + ": " + data.scores.join(", ");
</script>

</body>
</html>

Objects and arrays can nest inside each other to any depth, which is what allows JSON to represent complex, real-world data like an order with a customer, a shipping address, and a list of line items.

Nested Structure

<!DOCTYPE html>
<html>
<body>

<h2>Result</h2>
<p id="demo"></p>

<script>
const data = {
  "orderId": 4471,
  "customer": { "name": "Priya", "vip": false },
  "items": [
    { "sku": "A100", "qty": 2 },
    { "sku": "B240", "qty": 1 }
  ]
};

document.getElementById("demo").innerHTML = "Order " + data.orderId + " for " + data.customer.name;
</script>

</body>
</html>

Commas and Trailing Commas

Commas separate entries in both objects and arrays, but JSON does not allow a trailing comma after the last entry — a common mistake when converting from JavaScript, where trailing commas are permitted.

RuleValidInvalid
Trailing comma{"a": 1, "b": 2}{"a": 1, "b": 2,}
Quoted keys{"a": 1}{a: 1}
String quotes{"a": "x"}{"a": 'x'}
Note: When debugging invalid JSON, check for trailing commas and unquoted keys first — they cause the vast majority of parse errors.

Exercise: JSON Syntax

In valid JSON, how must an object's keys (names) be written?

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