JSON Rules

Definition and Usage

JSON is a strict text format. A few hard rules decide whether JSON.parse accepts your document or throws SyntaxError.

Those rules are tighter than JavaScript object literals: always double-quote keys and strings, never leave a trailing comma, and stick to the six value types.

Core Rules

• Quoted keys Object keys must be double-quoted strings — "name", not name.
• Double quotes Strings use " only. Single quotes are invalid in JSON.
• Commas Separate properties and array items with commas — but not after the last one.
• No trailing comma {"a":1,} and [1,2,] fail.
• No comments // and /* */ are not allowed in strict JSON.
• Six types only string, number, object, array, boolean, null — no undefined, Date, or function.

Valid vs Invalid

Same idea: one document follows the rules; the other looks like a JS literal.

{
  "name": "Ada",
  "skills": ["math", "code"],
  "active": true
}

{
  name: 'Ada',
  skills: ['math', 'code',],
  active: true,
}

Rules Come First

JSON Schema describes shape and types after the text is valid JSON. If the document fails the rules above, parsers never reach schema checks.

Example: first the payload must parse; then a schema can require name and age.

{
  "type": "object",
  "required": ["name", "age"],
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "integer" }
  }
}

Examples

Use View Output for a quick result, or Try It Yourself to edit and run in the browser.

const text = `{
  "name": "Ada",
  "skills": ["math", "code"],
  "active": true
}`;

const data = JSON.parse(text);
console.log(data.name);
console.log(data.skills.length);
console.log(data.active);

Ada
2
true

const bad = '{"name":"Ada","age":36,}';

try {
  JSON.parse(bad);
  console.log("Parsed OK");
} catch (err) {
  console.log(err.name);
  console.log("Trailing comma is invalid");
}

SyntaxError
Trailing comma is invalid

const ok = '{"name":"Ada"}';
const bad = "{'name':'Ada'}";

console.log(JSON.parse(ok).name);

try {
  JSON.parse(bad);
} catch (err) {
  console.log(err.name + ": use double quotes");
}

Ada
SyntaxError: use double quotes