isJSON

Checks if a given value is a valid JSON string.

A valid JSON string is one that can be successfully parsed using JSON.parse(). According to JSON specifications, valid JSON can represent:

• Objects (with string keys and valid JSON values)
• Arrays (containing valid JSON values)
• Strings
• Numbers
• Booleans
• null

String values like "null", "true", "false", and numeric strings (e.g., "42") are considered valid JSON and will return true.

This function serves as a type guard in TypeScript, narrowing the type of the argument to string.

Interface

function isJSON(value: unknown): value is string;

Parameters

• value (unknown): The value to check if it's a valid JSON string.

Return Value

(value is string): Returns true if the value is a valid JSON string, otherwise false.

Examples

import { isJSON } from 'es-toolkit/predicate';

const value1 = '{"name":"John","age":30}';
const value2 = '[1,2,3]';
const value3 = 'true';
const value4 = 'null';
const value5 = '42';
const value6 = 'invalid json';
const value7 = { name: 'John' };
const value8 = null;

console.log(isJSON(value1)); // true
console.log(isJSON(value2)); // true
console.log(isJSON(value3)); // true (parsed as boolean per JSON spec)
console.log(isJSON(value4)); // true (parsed as null per JSON spec)
console.log(isJSON(value5)); // true (parsed as number per JSON spec)
console.log(isJSON(value6)); // false
console.log(isJSON(value7)); // false (not a string)
console.log(isJSON(value8)); // false (not a string)