js作用域链与预解析

1     <script>
2         var a = "全局变量a"
3         let b = "全局变量b"
4         const c = "全局变量c"
5     </script>

1function aaa(params) {
2  var bb = "bb";
3}
4
5console.log(bb);//bb is not defined

1if (true) {
2  var a = "a";
3  let b = "b";
4  const c = "c";
5}
6
7console.log(a);//"a"
8console.log(b);//b is not defined
9console.log(c);//c is not defined
10

1var bb = "bb"; //在这层找到了
2
3(function aaa() {
4  //当前作用域没有，向上层查找
5  console.log(bb);
6})(); //bb