# 2.2 Array 操作方法

* **Array.forEach(function(element) {})**

  將陣列內的元素抓取出來，並可對其操作。

  ```
  var arr = [1, 2, 3];
  arr.forEach(function(element) {
    console.log(element);    // 1 2 3
  });
  ```
* **Array.map(function(element) {})**

  將陣列內的元素抓取出來，並可對其操作，並在function執行結束後回傳一個新的陣列。

  ```
  var arr = [1, 2, 3];
  var arr2 = arr.map(function(element) {
    return element * 2;
  });
  console.log(arr);    // [1, 2, 3]
  console.log(arr2);   // [2, 4, 6]
  ```
* **Array.filter(function(element) {})**

  過濾陣列內的元素，並回傳過濾後的新陣列。

  ```
  var arr = [1, 2, 3];
  var arr2 = arr.filter(function(element) {
    return element < 3;
  });
  console.log(arr2);     // [1, 2]
  ```
* **Array.reduce(function(accumulator, currentValue) {})**

  將陣列內的元素全部加總。
* ```
  var arr = [1, 2, 3];
  var sum = arr.reduce(function(accumulator, currentValue) {
    // accumulator 前一個回傳值
    return accumulator += currentValue;
  });
  console.log(sum);    // 6
  ```
* **Array.indexOf(element)**

  抓取陣列內元素的index。

  ```
  var arr = [1, 2, 3];
  var index = arr.indexOf(1);
  console.log(index);     // 0
  ```


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