const:
const声明的变量必须直接初始化且不允许二次赋值
<script type="text/javascript">
const name="Tom";
console.log(name);
name="Tim";//对name进行了二次赋值,将会报错
console.log(name);
</script>
报错内容:TypeError: invalid assignment to const `name’
let:
和var一样,let也用于声明变量
let和var的区别:
一、在全局作用域中,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
<script type="text/javascript">
let address="河南省郑州市高新区";
console.log(address);
console.log(window.address);//undefined
</script>
<script type="text/javascript">
var address="河南省郑州市高新区";
console.log(address);
console.log(window.address);
</script>
二、在函数作用域中:和var一样,都用于声明变量,此时两者没有区别
三、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>
四、let不能重复声明变量,var则可以重复声明
<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