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find() 方法會回傳第一個滿足所提供之測試函式的元素。否則回傳 undefined

也可以參考 findIndex() 方法,它回傳被找到的元素在陣列中的索引,而不是它的值。

If you need to find the position of an element or whether an element exists in an array, use Array.prototype.indexOf() or Array.prototype.includes().

語法

arr.find(callback[, thisArg])

參數

callback
會處理陣列中每個元素的函數,它使用三個參數:
element
在陣列中正被處理的元素。
index選擇性
在陣列中正被處理的元素的索引。
array選擇性
呼叫 find 的陣列。
thisArg 選擇性
執行 callback 函式時被當作 this 的物件。

回傳值

若元素通過測試則為其值;否則為 undefined

描述

find 方法會對每個元素執行一次 callback 函式,直到找到一個讓 callback 函式回傳 true 的元素。當元素被找到的時候,find 會立刻回傳該元素,否則 find 會回傳 undefinedcallback 會被使用於陣列索引自 0length - 1,並會被用於每一個的陣列索引,而不僅是那些有賦值的索引。這代表此方法在稀疏陣列(sparse arrays)上的效能可能較其他只存取已賦值索引的方法來的差。

callback 函式被呼叫時會傳入三個參數:元素的值、元素索引,以及正被迭代的陣列物件。

如果提供 thisArg 參數予 find,其將會被當作 callback 每次被呼叫的 this。若是沒提供,則會使用 undefined

find 並不會改變呼叫該方法的陣列。

The range of elements processed by find is set before the first invocation of callback. Elements that are appended to the array after the call to find begins will not be visited by callback. If an existing, unvisited element of the array is changed by callback, its value passed to the visiting callback will be the value at the time that find visits that element's index; elements that are deleted are still visited.

範例

Find an object in an array by one of its properties

var inventory = [
    {name: 'apples', quantity: 2},
    {name: 'bananas', quantity: 0},
    {name: 'cherries', quantity: 5}
];

function isCherries(fruit) { 
    return fruit.name === 'cherries';
}

console.log(inventory.find(isCherries)); 
// { name: 'cherries', quantity: 5 }

在陣列中找質數

以下範例在陣列中找出一個屬於質數的元素,如果裡面不含質數則回傳 undefined

function isPrime(element, index, array) {
  var start = 2;
  while (start <= Math.sqrt(element)) {
    if (element % start++ < 1) {
      return false;
    }
  }
  return element > 1;
}

console.log([4, 6, 8, 12].find(isPrime)); // undefined, not found
console.log([4, 5, 8, 12].find(isPrime)); // 5

The following examples show that non-existent and deleted elements are visited and that the value passed to the callback is their value when visited.

// Declare array with no element at index 2, 3 and 4
var a = [0,1,,,,5,6];

// Shows all indexes, not just those that have been assigned values
a.find(function(value, index) {
  console.log('Visited index ' + index + ' with value ' + value); 
});

// Shows all indexes, including deleted
a.find(function(value, index) {

  // Delete element 5 on first iteration
  if (index == 0) {
    console.log('Deleting a[5] with value ' + a[5]);
    delete a[5];
  }
  // Element 5 is still visited even though deleted
  console.log('Visited index ' + index + ' with value ' + value); 
});

Polyfill

這個方法在 ECMAScript 2015 中首次被規範,可能尚未在所有 JavaScript 應用中被實作。你可以使用以下程式片段來 polyfill Array.prototype.find

// https://tc39.github.io/ecma262/#sec-array.prototype.find
if (!Array.prototype.find) {
  Object.defineProperty(Array.prototype, 'find', {
    value: function(predicate) {
     // 1. Let O be ? ToObject(this value).
      if (this == null) {
        throw new TypeError('"this" is null or not defined');
      }

      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0;

      // 3. If IsCallable(predicate) is false, throw a TypeError exception.
      if (typeof predicate !== 'function') {
        throw new TypeError('predicate must be a function');
      }

      // 4. If thisArg was supplied, let T be thisArg; else let T be undefined.
      var thisArg = arguments[1];

      // 5. Let k be 0.
      var k = 0;

      // 6. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kValue be ? Get(O, Pk).
        // c. Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
        // d. If testResult is true, return kValue.
        var kValue = o[k];
        if (predicate.call(thisArg, kValue, k, o)) {
          return kValue;
        }
        // e. Increase k by 1.
        k++;
      }

      // 7. Return undefined.
      return undefined;
    }
  });
}

If you need to support truly obsolete JavaScript engines that don't support Object.defineProperty, it's best not to polyfill Array.prototype methods at all, as you can't make them non-enumerable.

規範

規範 狀態 備註
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Array.prototype.find' in that specification.
Standard 首次定義
ECMAScript Latest Draft (ECMA-262)
The definition of 'Array.prototype.find' in that specification.
Draft  

瀏覽器相容性

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support45 Yes25 No328
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes4 Yes8 Yes

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