1. 任何变量，如果未经声明就赋值，此变量是属于 window 的属性，而且不会做变量提升。（注意，无论在哪个作用域内赋值）

function foo() {
    var a = (b = 100); // a和b的区别
}

2. defer and async

   The best thing to do to speed up your page loading when using scripts is to put them in the head, and add a defer attribute to your script tag.

   Efficiently load JavaScript with defer and async