And calls this function

Function internal attributes:

　　[[call]]  [[caller]]

Original understand this:

1 function hell( thing ){
2   console.log( this + " say " + thing );  
3 }
4 
5 hello.call( "LiLei","hello" ); // LiLei say hello  (指向 LiLei)
6 hello( "hello" ); //say hello  （指向 window）
7 
8 es6 严格模式：
9     this undefined