You do not know the JS series (25) - an indirect reference

You may have intentionally or unintentionally create a function of "indirect references", in this case, the application calls this function will default rule

Indirect references most likely to occur when the assignment:

function foo(){
　　console.log(this.a);
}
var a = 2;
var o = { a: 3, foo: foo};
var p = { a: 4 };
o.foo(); // 3
(p.foo = o.foo)(); // 2

Assignment expression p.foo = o.foo return value is a reference to the objective function, so the position is to call foo (), not o.foo (). According to default binding implicit binding said before, there will be applications