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普通变量声明:
<script type="text/javascript">
alert(a); //报错 a is not defined
var a = 1;
</script>
变量提升:
<script type="text/javascript">
alert(a); //undefined if语句的变量提升
if(1<2){
var a = 1;
}
box();
function box(){
alert(b); //undefined 函数的变量提升
if(1<2){
var b = 2;
}
}
</script>