For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt, and this page is available as Markdown at /reference/predicate/isJSON.md.
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    isJSON es-toolkit

    Checks if a value is a valid JSON string.

    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.

    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:

    // 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:

    // 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:

    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

    valueunknownrequired

    The value to check if it's a valid JSON string.

    Returns

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

    value is string
    Source: es-toolkit

    Re-exported verbatim from es-toolkit. Implementation, edge cases, and performance behavior are owned upstream. This page mirrors the documentation at the pinned version; the linked source is authoritative.