Javascript type checking

typechecks.js

var typeOf = function( type ) {
    return function( value ) {
        if ( typeof value !== type ) {
            throw new TypeError( 'Expected a ' + type + '!' );
        } else {
            return value;
        }
    };
};

var num = typeOf( 'number' );
var bool = typeOf( 'boolean' );
var str = typeOf( 'string' );
var func = typeOf( 'function' );
var undef = typeOf( 'undefined' );
var obj = typeOf( 'object' );

console.log(num(3)); // 3
console.log(num("3")); //Error: Expected a Number.

var is_array = function(arr) {
  if ( {}.toString.call(arr) !== '[object Array]' ) {
    throw new TypeError( 'Expected an Array!' );
  } else {
    return arr;
  }
};

console.log(is_array([1,2,3])); //[1,2,3]
console.log(is_array(4); //Error: Expected an Array