Array.prototype.map()

map() , dizinin her elemanı için, parametre olarak verilen fonksiyonu çağırır ve oluşan sonuçlarla da yeni bir dizi oluşturur.

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

callback

Yeni oluşan dizinin elemanlarını döndürdüğü değerlerle oluşturur, üç parametre alır:

 

currentValue

Dizinin o anda işlem yapılan elemanı.

indexOptional

Dizinin o anda işlem yapılan elemanının indeksi.

arrayOptional

map tarafından çağırılan dizi.

thisArgOptional

callback fonsiyonu çalışırken kullanacağı this değeri.

Elemanları, callback fonksiyonunun sonuçları olan yeni bir dizi.

map, kendisine özel tahsis edilmiş geri çağırma (callback) fonksiyonunu çağırarak, sıraya uygun olarak döngüye giren her eleman için sonuçtan yeni bir dizi(array) inşa eder. callback sadece değere tanımlanmış dizinin indeksleri için tetiklenir, undefined de buna dahildir. Dizinin kayıp elemanları için çağrılmaz ( kayıp elemanlar: silinmiş veya hiçbir değere eşitlenmemiş veya oluşmamış indeksleri ifade eder. )

callback şu üç argüman ile tetiklenir; Elemanın değeri, elemanın indeks numarası ve elemanları gezilecek olan dizi objesi...

dizi.map( (deger, index) => /* yapılacak işlem */ ) 

If a thisArg parameter is provided to map, it will be used as callback's this value. Otherwise, the value undefined will be used as its this value. The this value ultimately observable by callback is determined according to the usual rules for determining the this seen by a function.

map does not mutate the array on which it is called (although callback, if invoked, may do so).

The range of elements processed by map is set before the first invocation of callback. Elements which are appended to the array after the call to map begins will not be visited by callback. If existing elements of the array are changed, their value as passed to callback will be the value at the time map visits them. Elements that are deleted after the call to map begins and before being visited are not visited.

Due to the algorithm defined in the specification if the array which map was called upon is sparse, resulting array will also be sparse keeping same indices blank.

The following code takes an array of numbers and creates a new array containing the square roots of the numbers in the first array.

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

The following code takes an array of objects and creates a new array containing the newly reformatted objects.

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

var reformattedArray = kvArray.map(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}]

The following code shows how map works when a function requiring one argument is used with it. The argument will automatically be assigned from each element of the array as map loops through the original array.

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]

This example shows how to use map on a String to get an array of bytes in the ASCII encoding representing the character values:

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]

This example shows how to iterate through a collection of objects collected by querySelectorAll. In this case we get all selected options on the screen and printed on the console:

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

Easier way would be using Array.from() method.

(inspired by this blog post)

It is common to use the callback with one argument (the element being traversed). Certain functions are also commonly used with one argument, even though they take additional optional arguments. These habits may lead to confusing behaviors.

// 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]

One alternative output of the map method being called with parseInt as a parameter runs as follows:

var xs = ['10', '10', '10'];

xs = xs.map(parseInt);

console.log(xs);
// Actual result of 10,NaN,2 may be unexpected based on the above description.

map was added to the ECMA-262 standard in the 5th edition; as such it may not be present in all implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of map in implementations which do not natively support it. This algorithm is exactly the one specified in ECMA-262, 5th edition, assuming Object, TypeError, and Array have their original values and that callback.call evaluates to the original value of Function.prototype.call.

// 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;
  };
}

• Array.prototype.forEach()
• Map object
• Array.from()