.JSON

Introduction

JSON is a text format for structured data, taken from the way JavaScript writes objects and arrays. Douglas Crockford described it in the early 2000s, and it is now standardised as ECMA-404 and RFC 8259. It is the usual format for web APIs and is widely used for settings files and data exchange between programs.

A related format, JSON Lines (.jsonl), stores one JSON value per line so that a large file can be read or appended a record at a time.

Software

• Visual Studio Code (Windows, macOS, Linux)
• Notepad++ (Windows)
• Sublime Text (Windows, macOS, Linux)
• Firefox, which shows JSON as a browsable tree (Windows, macOS, Linux)
• jq, a command-line JSON processor (Windows, macOS, Linux)
• Python and Node.js, which read and write JSON with no extra libraries

Structure

A JSON file is UTF-8 text holding one value, usually an object or an array. There is no header and no magic number. A value is one of six kinds:

• an object: { } around comma-separated "name": value pairs
• an array: [ ] around comma-separated values
• a string: double quotes, with backslash escapes such as \n, \" and é
• a number: decimal, with an optional fraction and exponent
• true or false
• null

Spaces and line breaks between values carry no meaning. JSON has no comments, no trailing commas and no way to write infinity or NaN. RFC 8259 says a file must not start with a byte order mark, although many readers accept one.