Array.prototype.map()

var numbers = [1, 4, 9];
var roots = numbers.map(Math.sqrt); //map會return一個新的array
// roots 現在是 [1, 2, 3]
/* numbers 還是 [1, 4, 9]，這證明了 map() 不會去變動到 numbers 的值，
   map 內部是做了 immutable 的機制，Array.prototype 底下的這些高階函式
   大多都具有這樣函數式編程裡非常注重的特性 - immutable，不會去改變資料
   來源本身原有的值
*/

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 現在是 [{1: 10}, {2: 20}, {3: 30}],

// kvArray 仍然是：
// [{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 現在是 [2, 8, 18]
// numbers 仍然是 [1, 4, 9]

var map = Array.prototype.map;
var a = map.call("Hello World", function (x) {
  return x.charCodeAt(0);
});
// a 現在等於 [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;
});

// Consider:
["1", "2", "3"].map(parseInt);
// 以為會是 [1, 2, 3] 嗎
// 其實是 [1, NaN, NaN]

// parseInt 通常只用上一個參數 argument，但他其實用了兩個：
// 第一個是表達式，第二個則是進位數。
// 對該回呼函式來說 Array.prototype.map 帶了三個參數：
// 元素、索引、陣列
// 第三個參數會被 parseInt 忽略，但它可不會忽略第二個，
// 因此可能造成困惑。可以去看上面提到的部落格文章以獲知詳情。

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;
  };
}