from You dont know js on github
To learn this, you first have to learn what this is not, despite any assumptions or misconceptions that may lead you down those paths. this is neither a reference to the function itself, nor is it a reference to the function’s lexical scope.
this is actually a binding that is made when a function is invoked, and what it references is determined entirely by the call-site where the function is called.
this 并不是代表函数本身。也不是函数词法作用域的参考。
this 在函数运行的时候被绑定,它参考什么只决定于函数的调用方式。

me: this 在单纯的函数声明方式中表示 Grobal Object,在作为对象的函数时指代对象。