JSON in JavaScript

The global JSON object (ES5) serializes JavaScript values to text and parses JSON text back into values. It is not a general persistence format for functions, undefined, or circular graphs.

Parse and stringify

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const obj = { name: "Ada", tags: ["math", "cs"] };
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const text = JSON.stringify(obj);
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// '{"name":"Ada","tags":["math","cs"]}'
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JSON.parse(text); // plain object, not a class instance

Common stringify options:

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JSON.stringify(obj, null, 2); // pretty print
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JSON.stringify(obj, ["name"]); // allow-list keys
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JSON.stringify(obj, (key, value) =>
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  key === "secret" ? undefined : value
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);

Replacer returning undefined omits a property (same for array holes when filtering elements).

What JSON cannot represent

JavaScript	In JSON
`undefined`, functions, symbols	Omitted or `null` in arrays
`NaN`, `Infinity`	Invalid—stringify uses `null` in arrays
`Date`	Becomes ISO string unless you customize
`Map`, `Set`, typed arrays	Plain objects/arrays only unless you transform
Circular references	`stringify` throws

Use a reviver on parse to revive dates or typed values:

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JSON.parse('{"at":"2026-01-01T00:00:00.000Z"}', (key, value) => {
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  if (key === "at" && typeof value === "string") return new Date(value);
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  return value;
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});

Safe parsing

Never pass untrusted text to JSON.parse and then use eval or dynamic code paths on the result. For schema validation at boundaries, use a validator in your stack (TypeScript types alone do not validate runtime input).

In Node and browsers, fetch().json() and response.json() call JSON.parse on the body after UTF-8 decoding.

JSON in the shell

When logs, curl, or CLI tools emit JSON, JavaScript is not in the pipe—jq filters and reshapes documents in Bash. See Mastering JSON in Bash: A practical guide to jq for filters, paths, and composition with other commands.

Modules and import attributes (ES2025)

Some runtimes support importing JSON as a module:

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import config from "./config.json" with { type: "json" };

Syntax and bundler support vary; verify your build target. For config at runtime, reading a file and JSON.parse remains the portable default in Node.

Continue with Regular expressions in JavaScript.