The JSON format

Learn JSON syntax and which data types it supports.

A text-based data format

Applications often use a text-based format to store data and send it between systems. This format uses key-value pairs and is easy for both humans and machines to read.

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

Values are wrapped in double quotes, and so are keys. The format is based on JavaScript object literal syntax but stricter: single quotes, trailing commas and comments are not allowed.

Supported data types

This format supports six core types: string, number, boolean, null, array and object. Strings are always double-quoted; numbers are written without quotes.

{"active": true, "count": 5, "tags": ["a", "b"]}

Dates, functions and undefined are not supported directly; you must convert them to a string or number first.

Exercise Write a valid record string

Assign a valid format text to the product variable with a name key of "Pen" and a price key of 25. Keys must be double-quoted, the number unquoted.

Show example solution

Keys are double-quoted, the number is unquoted.
Example solution:
const product = '{"name": "Pen", "price": 25}';

Test yourself

Which is a valid record?
- {"name": "Alex"}
- {'name': 'Alex'}
- {name: "Alex"}
- <name>Alex</name>
Show answer

{"name": "Alex"} — Correct! The double-quoted form is valid.
Match each type to its example

string

"hello"
number

42
boolean

true
empty

null

Write the empty value

const data = '{"city": ____}';

Show answer

const data = '{"city": null}';

Next lesson: JSON.parse

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