isJSON

Checks if a value is a valid JSON string.

typescript

const result = isJSON(value);

Usage

`isJSON(value)`

Use isJSON when you want to check if a string is a valid JSON format. This function verifies if it can be parsed with JSON.parse(). According to JSON specifications, strings representing objects, arrays, strings, numbers, booleans, and null are all valid.

typescript

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

// Valid JSON strings
console.log(isJSON('{"name":"John","age":30}')); // true
console.log(isJSON('[1,2,3]')); // true
console.log(isJSON('"hello world"')); // true
console.log(isJSON('42')); // true
console.log(isJSON('true')); // true
console.log(isJSON('false')); // true
console.log(isJSON('null')); // true

// Invalid JSON strings
console.log(isJSON('undefined')); // false
console.log(isJSON('function() {}')); // false
console.log(isJSON('{name: "John"}')); // false (keys without quotes)
console.log(isJSON("{'name': 'John'}")); // false (single quotes)
console.log(isJSON('{}')); // true (empty object is valid)
console.log(isJSON('[]')); // true (empty array is valid)

All non-string values return false:

typescript

// Non-string values
console.log(isJSON({ name: 'John' })); // false
console.log(isJSON([1, 2, 3])); // false
console.log(isJSON(42)); // false
console.log(isJSON(true)); // false
console.log(isJSON(null)); // false
console.log(isJSON(undefined)); // false

Useful for API response or user input validation:

typescript

// API response validation
function processApiResponse(response: unknown) {
  if (isJSON(response)) {
    try {
      const data = JSON.parse(response);
      console.log('Parsed data:', data);
      return data;
    } catch (error) {
      // Won't execute here since isJSON returned true
      console.error('Parsing failed:', error);
    }
  }

  console.log('Not a valid JSON string');
  return null;
}

// User input validation
function validateJsonInput(input: unknown): string | null {
  if (isJSON(input)) {
    // TypeScript infers input as string
    return input;
  }

  throw new Error('Input must be a valid JSON string');
}

// Configuration file validation
function loadConfig(configString: unknown) {
  if (isJSON(configString)) {
    const config = JSON.parse(configString);
    return {
      isValid: true,
      config,
      error: null,
    };
  }

  return {
    isValid: false,
    config: null,
    error: 'Invalid JSON format',
  };
}

Accurately detects complex JSON structures:

typescript

const complexJson = `{
  "users": [
    {
      "id": 1,
      "name": "Alice",
      "preferences": {
        "theme": "dark",
        "notifications": true
      }
    }
  ],
  "meta": {
    "total": 1,
    "page": 1
  }
}`;

console.log(isJSON(complexJson)); // true

// Invalid formats
console.log(isJSON('{ "name": "John", }')); // false (trailing comma)
console.log(isJSON('{ name: "John" }')); // false (unquoted key)
console.log(isJSON("{ 'name': 'John' }")); // false (single quotes)

Parameters

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

Returns

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