---
tags: Array, ES6, Javascript
disqus: hackmd
---

# [JS]Array.prototype.find()

>ie不支援

[Array.prototype.find() - MDN](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Reference/Global_Objects/Array/find)
[IE alternative to Array.prototype.find()](https://stackoverflow.com/questions/43239598/ie-alternative-to-array-prototype-find)

---

關於IE不支援的問題，除了MDN的polyfill外，stackoverflow也有人提出其他方法。

### MDN的polyfill
先看看MDN的polyfill，MDN也提到如果你的環境不支援[Object.defineProperty()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty)，那就不要用這個polyfill。

// 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;
    }
  });
}

### 自己寫一個function

find返回符合的第一筆資料。
所以當使用for迴圈的時候，當一符合指定就返回資料並且return結束。

var data = [{id: 1, name: 'a'}, {id: 2, name: 'b'}];

function altFind(arr, callback) {
    for (var i = 0; i < arr.length; i++) {
        var match = callback(arr[i], i);
        if (match) {
            return arr[i];
        }
    }
}

var result = altFind(data, function(e, index) {
    console.log('index', index);
    return e.id == 2;
});

console.log(result);