JSON Syntax

JSON has only a handful of rules, and a parser enforces every one of them. Break a single rule and the whole document is rejected. This lesson lists the rules, shows the mistakes that break them most often, and gives you a small function to test whether any text is valid JSON.

// The Basic Shape

JSON data is built from two structures: an object (curly braces holding named values) and an array (square brackets holding a list). Everything else is a plain value inside them.

{
  "title": "Dune",
  "pages": 412,
  "authors": ["Frank Herbert"],
  "inPrint": true
}

• Data is written as name: value pairs, with a colon between the name and the value
• Pairs are separated by commas
• Curly braces { } hold an object
• Square brackets [ ] hold an array

// Rule 1: Keys Are Double-Quoted Strings

Every key must be a string in double quotes. A JavaScript object lets you write title: "Dune", but JSON does not: it must be "title": "Dune". Single quotes are not allowed anywhere, for keys or for values.

// Rule 2: Strings Use Double Quotes

A string value is also written in double quotes. To put special characters inside, use a backslash escape:

const text = '"She said \\"hi\\"\\nThen left."';
console.log(JSON.parse(text));

The doubled backslashes in the JavaScript line above are there for JavaScript itself: inside a JS string, \\n is a backslash followed by an n, which is exactly what JSON needs to see.

// Rule 3: Commas Between Items, Never After the Last

{ "a": 1, "b": 2 }       valid
{ "a": 1, "b": 2, }      invalid: comma after the last item
[1, 2, 3]                 valid
[1, 2, 3,]                invalid

JavaScript lets you leave a trailing comma, and many people add one by habit. JSON does not.

// Rule 4: No Comments, No Extras

JSON has no comment syntax at all: no //, no /* */. It also has no undefined, no functions, and no way to say "this value is a Date". Only the six kinds of values from the next lesson are allowed.

// Whitespace Does Not Matter

Spaces, tabs, and line breaks between pieces are ignored. These two documents are identical to a parser:

{"name":"Ava","age":16}

{
  "name": "Ava",
  "age": 16
}

Servers often send the compact version to save bytes. People usually format it with indentation so it is readable.

// The Top Level Can Be Any Value

A JSON document does not have to be an object. A whole document can be an array, a string, a number, true, false, or null:

console.log(JSON.parse("[1, 2, 3]"));
console.log(JSON.parse('"hello"'));
console.log(JSON.parse("42") + 1);
console.log(JSON.parse("null"));

// Testing Whether Text Is Valid

The easiest way to check JSON is to try to parse it. If the text breaks a rule, JSON.parse throws a SyntaxError, so a small try...catch function can answer yes or no (JS Errors covers try...catch):

function isValid(text) {
  try {
    JSON.parse(text);
    return true;
  } catch (error) {
    return false;
  }
}

console.log(isValid('{"name": "Ava"}'));
console.log(isValid("{name: 'Ava'}"));
console.log(isValid('{"a": 1,}'));
console.log(isValid('{"a": undefined}'));
console.log(isValid('// note\n{"a": 1}'));

// The Mistakes That Break It

// Quick Check

console.log(JSON.parse("42") + 1);

// Try It

▶ Open Playground

1. Click Open Playground and run it. Predict each true or false before you look
2. Fix this broken JSON until isValid says true: {name: 'Ava', "grade": 10,}
3. Write a valid JSON array that contains a number, a string, and null, then parse it
4. Add a comment to valid JSON and confirm the parser rejects it