.json File Extension

What kind of file is a .json?

A .json file stores JavaScript Object Notation, a compact text format for exchanging structured data. It normally uses the application/json MIME type. JSON can represent objects, arrays, strings, numbers, true, false, and null. Despite its name and JavaScript-inspired syntax, it is language-independent.¹

JSON has no magic number. Valid content may start with { or [, but a complete JSON document may also be a string, number, Boolean, or null after optional whitespace. The reliable check is to decode the text and parse it with a JSON parser, not merely inspect its first character.¹

How do I open a .json on my computer?

Any text editor can display JSON on Windows, macOS, or Linux. A code editor such as Visual Studio Code is more useful for editing because it provides formatting, folding, validation, and schema-aware suggestions.² On a terminal, jq can pretty-print, filter, and transform JSON.³

Web browsers and mobile text editors can also display many .json files. For machine use, load the file with a JSON parser provided by your programming language. Do not pass untrusted JSON to eval() or another code-execution function. JSON is data, and treating it as executable code creates an avoidable security risk.¹

Poly support for .json

Poly opens JSON in a code-friendly text preview with syntax highlighting and find-in-file support. Keys and values are indexed for exact and semantic search, so you can find a configuration or exported record by its contents as well as its filename. The Poly agent can read the text and reason about the data without running scripts referenced by it.

Parsing proves only that the syntax is valid. It does not prove that required fields exist or values match an application's schema. Use a JSON Schema validator when the receiving system requires a particular structure.

History of .json

JSON was presented publicly at JSON.org in 2001. Ecma describes it as a lightweight, text-based syntax derived from ECMAScript but independent of any programming language.⁴ RFC 4627 registered application/json and the .json extension in 2006. The current Internet Standard, RFC 8259, was published in 2017.¹

For interoperable exchange between systems, RFC 8259 requires UTF-8. It registers no magic number and no charset parameter. Senders must not add a byte order mark, although parsers may choose to ignore one.¹

Working with .json and alternatives

JSON is a strong default for APIs, application data, and configuration that must work across languages. Keep these tradeoffs in mind:

Standard JSON does not allow comments or trailing commas. Duplicate object names can also be handled inconsistently by parsers, and large numbers may lose precision in software that uses IEEE 754 binary64 values.¹ For predictable protocols, I-JSON narrows JSON with stricter rules for names, numbers, and Unicode.⁵

Choose CBOR when binary data, compact encoding, or richer native types matter. CBOR can represent the JSON data model, but tags, byte strings, and non-string map keys need an agreed mapping when converted back to JSON.⁶ Changing the filename is never enough; use a parser and serializer for the destination format.