前端基础-ECMAScript块级作用域

//注意 块级作用域中，使用var声明的变量是全局变量
{
	var a = 1;
	console.log(a);//1
}
console.log(a);//1

if(true){
	var b = 2;
	console.log(b);//2
}
console.log(b);//2

<script>
    if(true){
        let i=0;
        console.log(i);
    }
  console.log(i);//报错
</script>
<script>
    for(let i=0; i<=6; ++i){
        console.log(i);
    }
    console.log(i);//报错
</script>

<script>
  	// let声明的变量可以重新赋值
    {
        let a = 1;
      	console.log(a);
      	a = 2;
     	console.log(a);;
    }
</script>

<script>
  // let声明的变量不能在同一作用域重新声明，直接报错 预解析错误
  {
  	let a = 1;
  	console.log(a);
  	let a = 2;
  	console.log(a);
}
</script>

{
 	console.log(i);//报错
	let i = 8;
}

var arr = [3,4,5,6,7];

for(let i=0; i<arr.length; i++){
	// (function(i){
		setTimeout(function(){
			console.log(i);
			//console.log(arr[i]);
		}, 1000);
	// })(i);
}