1. Function.prototype.defer = function(ms) {
    2.     //'this' is the one that calls defer----user.sayHi
    3.     let f = this;
    4.     return function(...args) {
    5.         //'this' is the one that calls the returned function
    6.         setTimeout(() => f.apply(this, args), ms);
    7.     }
    8. };
    9. //user.sayHi is not an entirety, regard them as separate things
    10. let user = {
    11.     name: "John",
    12.     sayHi() {
    13.         alert(this.name);
    14.     }
    15. }
    16. // user's sayHi is assigned a new function, which was returned by defer
    17. user.sayHi = user.sayHi.defer(1000);
    18. //sayHi now == return function(...args) ....
    19. //so 'this' is user
    21. user.sayHi();