var的缺点
1、可以重复声明
javascript"> var a = 12
var a = 5
alert(a) //52、无法限制修改（没有常量）
javascript"> var a = 12
a = 5
alert(a) //53、没有块级作用域
javascript"> if(true){
var a = 5
}
alert(a) //5不可重复声明 let、const
Identifier 'a' has already been declared
javascript"> let a = 5
let a = 12
alert(a) //Identifier 'a' has already been declared值不可修改 const
Assignment to constant variable
javascript"> const a = 12
a = 5
alert(a) //Assignment to constant variablejavascript"> let a = 12
a = 5
alert(a) // 5块级作用域 let、const
a is not defined
javascript"> if(true){
let a = 5
}
alert(a) //a is not defined实例、块级作用域的优势
经典点击案例
<body>
<input type="button" value="按钮">
<input type="button" value="按钮">
<input type="button" value="按钮">
<script>
window.onload = function () {
var aBtn = document.getElementsByTagName('input')
for (var i = 0; i < aBtn.length; i++) {
(function (i) {
aBtn[i].onclick = function () {
alert(i) //0 1 2
}
})(i);
}
}
</script>
</body>优化后
javascript"> window.onload = function () {
var aBtn = document.getElementsByTagName('input')
for (let i = 0; i < aBtn.length; i++) {
aBtn[i].onclick = function () {
alert(i)
}
}
}
