array.map

var numbers = [1, 5, 10, 15];
var doubles = numbers.map(function(x) {
   return x * 2;
}
// doubles is now [2, 10, 20, 30]
// numbers is still [1, 5, 10, 15]

var numbers = [1, 4, 9];
var roots = numbers.map(Math.sqrt
// roots is now [1, 2, 3]
// numbers is still [1, 4, 9]

var new_array = arr.map(function callback(currentValue, index, array) {
    // Return element for new_array
}[, thisArg])

var numbers = [1, 4, 9];
var roots = numbers.map(Math.sqrt
// roots is now [1, 2, 3]
// numbers is still [1, 4, 9]

var kvArray = [{key: 1, value: 10}, 
               {key: 2, value: 20}, 
               {key: 3, value: 30}];

var reformattedArray = kvArray.map(function(obj) { 
   var rObj = {};
   rObj[obj.key] = obj.value;
   return rObj;
}

// reformattedArray is now [{1: 10}, {2: 20}, {3: 30}], 

// kvArray is still: 
// [{key: 1, value: 10}, 
//  {key: 2, value: 20}, 
//  {key: 3, value: 30}]

var numbers = [1, 4, 9];
var doubles = numbers.map(function(num) {
  return num * 2;
}

// doubles is now [2, 8, 18]
// numbers is still [1, 4, 9]

var map = Array.prototype.map;
var a = map.call('Hello World', function(x) { 
  return x.charCodeAt(0 
}
// a now equals [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100]

var elems = document.querySelectorAll('select option:checked'
var values = Array.prototype.map.call(elems, function(obj) {
  return obj.value;
}

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

// Consider:
['1', '2', '3'].map(parseInt
// While one could expect [1, 2, 3]
// The actual result is [1, NaN, NaN]

// parseInt is often used with one argument, but takes two.
// The first is an expression and the second is the radix.
// To the callback function, Array.prototype.map passes 3 arguments: 
// the element, the index, the array
// The third argument is ignored by parseInt, but not the second one,
// hence the possible confusion. See the blog post for more details

function returnInt(element) {
  return parseInt(element, 10
}

['1', '2', '3'].map(returnInt // [1, 2, 3]
// Actual result is an array of numbers (as expected)

// Same as above, but using the concise arrow function syntax
['1', '2', '3'].map( str => parseInt(str) 

// A simpler way to achieve the above, while avoiding the "gotcha":
['1', '2', '3'].map(Number // [1, 2, 3]
// but unlike `parseInt` will also return a float or (resolved) exponential notation:
['1.1', '2.2e2', '3e300'].map(Number // [1.1, 220, 3e+300]

// Production steps of ECMA-262, Edition 5, 15.4.4.19
// Reference: http://es5.github.io/#x15.4.4.19
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 (arguments.length > 1) {
      T = arguments[1];
    }

    // 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,
        //   Writable: true,
        //   Enumerable: true,
        //   Configurable: true },
        // and false.

        // In browsers that support Object.defineProperty, use the following:
        // Object.defineProperty(A, k, {
        //   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;
  };
}