why Array.prototype.map.call instead of Array.map.call

Array is a constructor function to create arrays.

If you type Array in browser console you will get a function definition, something like

function Array() { [native code] }

While if you type Array.prototype in browser console you will get an empty array i.e [ ] i.e. an Array object.

Consider this excerpt

function a(){
console.log('hi');
function b(){console.log('b');}
function c(){console.log('c');}
return {b:b,c:c,d:this}
}

When you type d = new a();
Then d is an object having two properties which are functions i.e. b and c and you can call

>> d.b() //logs b
>> d.c() //logs c

But you cannot call

a.b() or a.c() // since function b and c is not property of a.

So just as function b and c are defined in function a. Similarly function map is defined in function Array.

So you cannot call Array.map() but you have to get an object of Array and call map function on it.
Array.prototype gives us an Array object
Therefore they are using Array.prototype.map.call(a,func)

http://stackoverflow.com/questions/20153455/why-array-prototype-map-call-instead-of-array-map-call 

Using map to reverse a string

var str = '12345';
Array.prototype.map.call(str, function(x) {
  return x;
}).reverse().join(''); 

// Output: '54321'
// Bonus: use '===' to test if original string was a palindrome

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map