# 章序号/节序号/节/笔记序号
codecademy, Condition
# 概念阐释
- 丰富`if...else` ，可以有更多的条件，让条件可以有多于**2种的可能**。
- 总是位于`if` 和`else`之间。
- `return` 不可以在除函数和switch中使用。
## 语法
`if`/`else if`/`else`
# 举例子
- **用编程语言：**
- 当stopLight等于red时，计算机评定为`false`，继续向下
- 当stopLight等于yellow时，计算机评定为`true`，blocked，现实结果"slow down"
- **用人话：**
- 红色停止
- 黄色减速
- 绿色通行
- 没有指示灯，注意！未知
# 类比、比较与对比
[[js condition switch 语句]]
# 问题
**1.**
*注意：字符串it's Spring 的 ' 怎么写？条件内用的是数学赋值运算符*
Let’s create a program that keeps track of the way the environment changes with the seasons. Write a conditional statement to make this happen!
In **main.js** there is already an `if...else` statement in place. Let’s add an `else if` statement that checks if `season` is equal to `'winter'`.
Inside the code block of the `else if` statement, add a `console.log()` that prints the string `'It\'s winter! Everything is covered in snow.'`.
**2.**
Add another `else if` statement that checks if `season` is equal to `'fall'`.
Inside the code block of the `else if` statement you just created, add a `console.log()` that prints the string `'It\'s fall! Leaves are falling!'`.
**3.**
Add a final `else if` statement that checks if `season` is equal to `'summer'`.
Inside the code block of the `else if` statement you just created, add a `console.log()` that prints the string `'It\'s sunny and warm because it\'s summer!'`.
```js
let season = 'summer';
if (season === 'spring') {
console.log('It\'s spring! The trees are budding!');
}
else {
console.log('Invalid season.');
}
```
# 问题答案
[codecademy](https://www.codecademy.com/courses/introduction-to-javascript/lessons/control-flow/exercises/else-if)
# 备注（经验集/错误集）
可以用[[js condition switch 语句]]代替
也可以写成函数
```js
function Light(stopLight){
switch(stopLight){
case 'red':
return 'stop';
break;
case'yellow':
return 'slow down';
break;
case 'green':
return 'go';
break;
default:
break;
}
};
console.log(Light('green'));
```