Array.prototype.filter()

filter() 方法，會建立一個在指定函數下，所有能通過該函數測試之元素的新陣列。

表達式

arr.filter(callback[, thisArg])

參數

callback

這個函式用來測試陣列中的每個元素，並可調用函式中的參數(element, index, array)。 若回傳值為true則將當前的元素保留; 若是false，則刪除該元素。

thisArg

Optional. Value to use as this when executing callback.

說明

filter() 會對陣列的每個元素，呼叫給定的 callback 函數，並且建立一個能令 callback 回傳 true 或是 等同於 true 值元素的新陣列。 callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values. Array elements which do not pass the callback test are simply skipped, and are not included in the new array.

callback is invoked with three arguments:

1. the value of the element
2. the index of the element
3. the Array object being traversed

If a thisArg parameter is provided to filter, it will be passed to callback when invoked, for use as its this value.  Otherwise, the value undefined will be passed for use 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.

filter() does not mutate the array on which it is called.

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

範例

範例：過濾所有的小數字

以下範例會用 filter() 建立一個把所有小於 10 的元素都移掉的陣列。

function isBigEnough(value) {
  return value >= 10;
}
var filtered = [12, 5, 8, 130, 44].filter(isBigEnough);
// filtered is [12, 130, 44]

範例：從 JSON 過濾無效的項目

以下範例會用 filter() 建立一個把非零 numeric id 的元素都過濾掉的的 JSON。

var arr = [
  { id: 15 },
  { id: -1 },
  { id: 0 },
  { id: 3 },
  { id: 12.2 },
  { },
  { id: null },
  { id: NaN },
  { id: 'undefined' }
];

var invalidEntries = 0;

function filterByID(obj) {
  if ('id' in obj && typeof(obj.id) === 'number' && !isNaN(obj.id)) {
    return true;
  } else {
    invalidEntries++;
    return false;
  }
}

var arrByID = arr.filter(filterByID);

console.log('Filtered Array\n', arrByID); 
// [{ id: 15 }, { id: -1 }, { id: 0 }, { id: 3 }, { id: 12.2 }]

console.log('Number of Invalid Entries = ', invalidEntries); 
// 4

Polyfill

filter() 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 filter() in ECMA-262 implementations which do not natively support it. This algorithm is exactly the one specified in ECMA-262, 5th edition, assuming that fn.call evaluates to the original value of Function.prototype.call(), and that Array.prototype.push() has its original value.

if (!Array.prototype.filter) {
  Array.prototype.filter = function(fun/*, thisArg*/) {
    'use strict';

    if (this === void 0 || this === null) {
      throw new TypeError();
    }

    var t = Object(this);
    var len = t.length >>> 0;
    if (typeof fun !== 'function') {
      throw new TypeError();
    }

    var res = [];
    var thisArg = arguments.length >= 2 ? arguments[1] : void 0;
    for (var i = 0; i < len; i++) {
      if (i in t) {
        var val = t[i];

        // NOTE: Technically this should Object.defineProperty at
        //       the next index, as push can be affected by
        //       properties on Object.prototype and Array.prototype.
        //       But that method's new, and collisions should be
        //       rare, so use the more-compatible alternative.
        if (fun.call(thisArg, val, i, t)) {
          res.push(val);
        }
      }
    }

    return res;
  };
}

規範

瀏覽器相容性

參見

• Array.prototype.forEach()
• Array.prototype.every()
• Array.prototype.some()
• Array.prototype.reduce()