73 lines
1.6 KiB
Markdown
73 lines
1.6 KiB
Markdown
|
---
|
|||
|
|
title: If-Else Statement
|
|||
|
|
localeTitle: If-Else声明
|
|||
|
|
---
|
|||
|
|
## 介绍
|
|||
|
|
|
|||
|
|
`if`指定的条件为`true` ， `if`语句执行语句。如果条件为`false` ，则可以使用`else`语句执行另一个语句。
|
|||
|
|
|
|||
|
|
**注意：** `else`语句是可选的。
|
|||
|
|
|
|||
|
|
```javascript
|
|||
|
|
if (condition)
|
|||
|
|
/* do something */
|
|||
|
|
else
|
|||
|
|
/* do something else */
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
可以链接多个`if...else`语句来创建`else if`子句。这指定了要测试的新条件，并且可以重复测试多个条件，检查直到执行真实语句。
|
|||
|
|
|
|||
|
|
```javascript
|
|||
|
|
if (condition1)
|
|||
|
|
/* do something */
|
|||
|
|
else if (condition2)
|
|||
|
|
/* do something else */
|
|||
|
|
else if (condition3)
|
|||
|
|
/* do something else */
|
|||
|
|
else
|
|||
|
|
/* final statement */
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
**注意：**如果要在`if` ， `else`或`else if`部分中执行多个语句， `if`语句周围需要花括号：
|
|||
|
|
|
|||
|
|
```javascript
|
|||
|
|
if (condition) {
|
|||
|
|
/* do */
|
|||
|
|
/* something */
|
|||
|
|
/* with multiple statements */
|
|||
|
|
} else {
|
|||
|
|
/* do something */
|
|||
|
|
/* else */
|
|||
|
|
}
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
[MDN链接](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/if…else) | [MSDN链接](https://msdn.microsoft.com/en-us/library/85yyde5c.aspx)
|
|||
|
|
|
|||
|
|
## 例子
|
|||
|
|
|
|||
|
|
**使用** `if...else` ：
|
|||
|
|
|
|||
|
|
```javascript
|
|||
|
|
// If x=5 z=7 and q=42. If x is not 5 then z=19.
|
|||
|
|
if (x == 5) {
|
|||
|
|
z = 7;
|
|||
|
|
q = 42
|
|||
|
|
else
|
|||
|
|
z = 19;
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
**使用** `else if` ：
|
|||
|
|
|
|||
|
|
```javascript
|
|||
|
|
if (x < 10)
|
|||
|
|
return "Small number";
|
|||
|
|
else if (x < 50)
|
|||
|
|
return "Medium number";
|
|||
|
|
else if (x < 100)
|
|||
|
|
return "Large number";
|
|||
|
|
else {
|
|||
|
|
flag = 1;
|
|||
|
|
return "Invalid number";
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
```
|