86 lines
2.1 KiB
Markdown
86 lines
2.1 KiB
Markdown
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---
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title: For Loop
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localeTitle: 对于循环
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---
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### 句法
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```javascript
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for ([initialization]); [condition]; [final-expression]) {
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// statement
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}
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```
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javascript `for`语句由三个表达式和一个语句组成：
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## 描述
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* 初始化 - 在循环上第一次执行之前运行。此表达式通常用于创建计数器。这里创建的变量的范围是循环。一旦循环完成它的执行就会被破坏。
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* condition - 在每次迭代执行之前检查的表达式。如果省略，则此表达式的计算结果为true。如果计算结果为true，则执行循环语句。如果计算结果为false，则循环停止。
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* final-expression - 每次迭代后运行的表达式。通常用于增加计数器。但它也可以用来递减计数器。
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* statement - 要在循环中重复的代码
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这三个表达式中的任何一个或语句都可以省略。 For循环通常用于计算一定数量的迭代以重复语句。在条件表达式求值为false之前，使用`break`语句退出循环。
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## 常见的陷阱
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**超过数组的边界**
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当多次索引数组时，很容易超出数组的边界（例如，尝试引用3元素数组的第4个元素）。
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```javascript
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// This will cause an error.
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// The bounds of the array will be exceeded.
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var arr = [ 1, 2, 3 ];
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for (var i = 0; i <= arr.length; i++) {
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console.log(arr[i]);
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}
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output:
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1
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2
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3
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undefined
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```
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有两种方法可以修复此代码。将条件设置为`i < arr.length`或`i <= arr.length - 1`
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### 例子
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迭代0-8的整数
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```javascript
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for (var i = 0; i < 9; i++) {
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console.log(i);
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}
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output:
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0
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1
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2
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3
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4
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5
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6
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7
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8
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```
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在条件表达式为false之前中断循环
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```javascript
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for (var elephant = 1; elephant < 10; elephant+=2) {
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if (elephant === 7) {
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break;
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}
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console.info('elephant is ' + elephant);
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}
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output:
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elephant is 1
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elephant is 3
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elephant is 5
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```
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### 其他资源
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* [MDN - 用于陈述](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for)
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