> For the complete documentation index, see [llms.txt](https://jack1in.gitbook.io/font-end/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jack1in.gitbook.io/font-end/2.-javascript/2.1-es6/2.1.1-var-let-const.md).

# 2.1.1 var、let、const

* **var**
* **let**
* **const**

`var` 是屬於 `函式作用域（function scope）` 

```
function test() {
  var str = 'Hello';
  console.log(str);
}
// Hello
test();

// str is not defined
console.log(str);
```

但是， `var` 因為是 `函式作用域` 的關係，所以在判斷式的區塊不受影響。

```
if (true) {
  var str = 'Hello';
}
// Hello
console.log(str);
```

這樣的用法，很容易造成 `區域變數` 覆蓋 `全域變數` 的問題。

`let` 是屬於 `區域作用域`

`const` 是屬於 `區域作用域`用來定義 `常數（constant）` ，只要宣告完以後就不能再更動了。


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