const、let、var关键字

const关键字

const声明的变量必须直接初始化且不允许二次赋值，如下代码：

<script type="text/javascript">
const name="Tom";
console.log(name);
name="Tim";//报错：TypeError: invalid assignment to const `name'
console.log(name);
</script>
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let关键字

和var一样，let也用于声明变量，那么var声明的变量和let声明的变量有何不同：

全局作用域

let声明的变量不会作为全局对象window的属性，而var声明的变量则可以，如下代码：

<script type="text/javascript">
let address="河南省郑州市高新区";
console.log(address);
console.log(window.address);//undefined
</script>
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<script type="text/javascript">
var address="河南省郑州市高新区";
console.log(address);
console.log(window.address);
</script>
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函数作用域

和var一样，都用于声明变量，此时两者没有区别：

<script type="text/javascript">
function print(){
var address="河南省郑州市高新区";
console.log(address);
}
print();
</script>
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<script type="text/javascript">
function print(){
let address="河南省郑州市高新区";
console.log(address);
}
print(http://www.my516.com);
</script>
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语句块作用域

let声明的变量只能用于语句块作用域内，而var声明的变量还可以在语句块作用域外使用

<script type="text/javascript">
if(true){
let address="河南省郑州市高新区";
console.log(address);
}
console.log(address);//ReferenceError: address is not defined
</script>
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<script type="text/javascript">
if(true){
var address="河南省郑州市高新区";
console.log(address);
}
console.log(address);
</script>
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重复声明

let不能重复声明变量，var则可以

<script type="text/javascript">
let address="河南省郑州市高新区";
let address = "吉林省长春市宽平区";
console.log(address);
</script>
错误信息：
SyntaxError: redeclaration of let address
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<script type="text/javascript">
var address="河南省郑州市高新区";
var address = "吉林省长春市宽平区";
console.log(address);//输出：吉林省长春市宽平区
</script>
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