Merge pull request loverajoel#423 from loverajoel/tip-79

Translate tip-79

_posts/zh_TW/javascript/2017-06-15-looping-over-arrays.md

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+---
+layout: post
+
+title: 循環陣列
+tip-number: 79
+tip-username: loverajoel
+tip-username-profile: https://github.com/loverajoel
+tip-tldr: 這裡有一些方法在 JavaScript 中可以循環陣列。我們將從典型例子並且補充標準規範。
+
+categories:
+    - zh_TW
+    - javascript
+---
+
+這裡有一些方法在 JavaScript 中可以循環陣列。我們將從典型例子並且補充標準規範。
+
+## while
+
+```javascript
+let index = 0;
+const array = [1,2,3,4,5,6];
+
+while (index < array.length) {
+  console.log(array[index]);
+  index++;
+}
+```
+
+## for（典型）
+
+```javascript
+const array = [1,2,3,4,5,6];
+for (let index = 0; index < array.length, index++) {
+  console.log(array[index]);
+}
+```
+
+## forEach
+
+```javascript
+const array = [1,2,3,4,5,6];
+
+array.forEach(function(current_value, index, array) {
+  console.log(`At index ${index} in array ${array} the value is ${current_value}`);
+});
+// => undefined
+```
+
+## map
+
+最後的 construct 很有用，然而，它不會回傳一個新的陣列，這可能對你指定的情況是不合乎的。`map` 透過對每個元素應用一個函式，並回傳新的陣列來解決問題。
+
+```javascript
+const array = [1,2,3,4,5,6];
+const square = x => Math.pow(x, 2);
+const squares = array.map(square);
+console.log(`Original array: ${array}`);
+console.log(`Squared array: ${array}`);
+```
+
+`map` 完整個宣告方式：`.map(current_value, index, array)`。
+
+## reduce
+
+根據 MDN：
+
+> reduce() 方法對累加器和數組中的每個元素（從左到右）應用一個函式，將其減少為一個值。
+
+```javascript
+const array = [1,2,3,4,5,6];
+const sum = (x, y) => x + y;
+const array_sum = array.reduce(sum, 0);
+console.log(`The sum of array: ${array} is ${array_sum}`);
+```
+
+
+## filter
+
+基於一個布林函式對陣列進行過濾。
+
+```javascript
+const array = [1,2,3,4,5,6];
+const even = x => x % 2 === 0;
+const even_array = array.filter(even);
+console.log(`Even numbers in array ${array}: ${even_array}`);
+```
+
+## every
+
+取得一個陣列，並且想要測試每個元素中是否滿足給定的條件？
+
+```javascript
+const array = [1,2,3,4,5,6];
+const under_seven = x => x < 7;
+
+if (array.every(under_seven)) {
+  console.log('Every element in the array is less than 7');
+} else {
+  console.log('At least one element in the array was bigger than 7');
+}
+```
+
+## some
+
+測試至少一個元素是否 match 我們的布林函式。
+
+```javascript
+const array = [1,2,3,9,5,6,4];
+const over_seven = x => x > 7;
+
+if (array.some(over_seven)) {
+  console.log('At least one element bigger than 7 was found');
+} else {
+  console.log('No element bigger than 7 was found');
+}
+```