Checking the types of function arguments in Javascript

Question

In Javascript, is there any way to check the types of a function's arguments? I want to write a function called checkTypes that does the following:

function checkTypes(typeArr){
    //if the types do not match typeArr, throw an error
}

function exampleUsage(arr1, arr2, num1){
    checkTypes("object", "object", "number");
    //throw an error if the types do not match the corresponding elements
}

Answer 1

Do not use typeof in this case. It's problematic for several reasons:

typeof null                 // 'object'
typeof []                   // 'object'
typeof 'foo'                // 'string'
typeof new String('foo')    // 'object'
'foo' == new String('foo')  // true

Instead, use Object::toString:

Object.prototype.toString.call(null)               // '[object Null]'
Object.prototype.toString.call([])                 // '[object Array]'
Object.prototype.toString.call('foo')              // '[object String]'
Object.prototype.toString.call(new String('foo'))  // '[object String]'

A decorator would meet your requirements:

var getType = function(value) {
  return Object.prototype.toString.call(value)
    .replace(/^\[object |\]$/g, '').toLowerCase();
};

var checkTypes = function(types, fn) {
  return function() {
    var args = Array.prototype.slice.call(arguments, 0);
    for (var idx = 0; idx < types.length; idx += 1) {
      var expected = types[idx];
      var received = getType(args[idx]);
      if (received != expected) {
        throw new TypeError('expected ' + expected + '; received ' + received);
      }
    }
    fn.apply(null, args);
  };
};

var exampleUsage = checkTypes(['array', 'array', 'number'], function(arr1, arr2, num1) {
  console.log('arr1:', arr1);
  console.log('arr2:', arr2);
  console.log('num1:', num1);
});

Usage examples:

exampleUsage([], [], 0);
// arr1: []
// arr2: []
// num1: 0

exampleUsage([], [], 'foo');
// TypeError: expected number; received string