# use strict

输出是什么？

```javascript
function getAge() {
  'use strict'
  age = 21
  console.log(age)
}

getAge()
```

{% tabs %}
{% tab title="选项" %}

* A: `21`
* B: `undefined`
* C: `ReferenceError`
* D: `TypeError`
  {% endtab %}

{% tab title="答案" %}
**答案: C**

使用 `"use strict"`，你可以确保不会意外地声明全局变量。我们从来没有声明变量 `age`，因为我们使用 `"use strict"`，它将抛出一个引用错误。如果我们不使用 `"use strict"`，它就会工作，因为属性 `age` 会被添加到全局对象中了。
{% endtab %}
{% endtabs %}


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