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

Mozilla has supplied an Array.map polyfill for unsupported browsers

if (!Array.prototype.map) {

  Array.prototype.map = function(callback, thisArg) {



    var T, A, k;



    if (this == null) {

      throw new TypeError( this is null or not defined);

    }



    // 1. Let O be the result of calling ToObject passing the |this| value as the argument.

    var O = Object(this);



    // 2. Let lenValue be the result of calling the Get internal method of O with the argument length.

    // 3. Let len be ToUint32(lenValue).

    var len = O.length >>> 0;



    // 4. If IsCallable(callback) is false, throw a TypeError exception.

    // See: http://es5.github.com/#x9.11

    if (typeof callback !== function) {

      throw new TypeError(callback +  is not a function);

    }



    // 5. If thisArg was supplied, let T be thisArg; else let T be undefined.

    if (thisArg) {

      T = thisArg;

    }



    // 6. Let A be a new array created as if by the expression new Array(len) where Array is

    // the standard built-in constructor with that name and len is the value of len.

    A = new Array(len);



    // 7. Let k be 0

    k = 0;



    // 8. Repeat, while k < len

    while(k < len) {



      var kValue, mappedValue;



      // a. Let Pk be ToString(k).

      //   This is implicit for LHS operands of the in operator

      // b. Let kPresent be the result of calling the HasProperty internal method of O with argument Pk.

      //   This step can be combined with c

      // c. If kPresent is true, then

      if (k in O) {



        // i. Let kValue be the result of calling the Get internal method of O with argument Pk.

        kValue = O[ k ];



        // ii. Let mappedValue be the result of calling the Call internal method of callback

        // with T as the this value and argument list containing kValue, k, and O.

        mappedValue = callback.call(T, kValue, k, O);



        // iii. Call the DefineOwnProperty internal method of A with arguments

        // Pk, Property Descriptor {Value: mappedValue, : true, Enumerable: true, Configurable: true},

        // and false.



        // In browsers that support Object.defineProperty, use the following:

        // Object.defineProperty(A, Pk, { value: mappedValue, writable: true, enumerable: true, configurable: true });



        // For best browser support, use the following:

        A[ k ] = mappedValue;

      }

      // d. Increase k by 1.

      k++;

    }



    // 9. return A

    return A;

  };      

}