1. // call
    2. Function.prototype.call = function(ctx, ...args) {
    3.     ctx = ctx || window;
    5.   const fnSymbol = Symbol('fn');
    7.   ctx[fnSymbol] = this;
    9.   ctx[fnSymbol](...args);
    11.   delete ctx[fnSymbol];
    12. }
    14. // test
    15. function fn(c, d) {
    16.     return this.a + c + d;
    17. }
    19. const obj = {
    20.     a: 1
    21. }
    23. fn.call(obj, 2, 3); // 6
    1. // apply
    2. Function.prototype.apply = function(ctx, args) {
    3.     ctx = ctx || window;
    5.   const fnSymbol = Symbol('fn');
    6.   ctx[fnSymbol] = this;
    8.   ctx[fnSymbol](...args);
    10.   delete ctx[fnSymbol];
    11. }
    13. // test
    14. function fn(c, d) {
    15.     return this.a + c + d;
    16. }
    18. const obj = {
    19.     a: 1
    20. }
    22. fn.apply(obj, [2, 3]); // 6
    1. // bind
    2. Function.prototype.bind = function(ctx, ...args) {
    3.     ctx = ctx || window;
    5.   const fnSymbol = Symbol('fn');
    6.   ctx[fnSymbol] = this;
    8.   return function (){
    9.     ctx[fnSymbol](...args);
    10.       delete ctx[fnSymbol];
    11.   }
    12. }
    14. // test
    15. function fn(c, d) {
    16.     return this.a + c + d;
    17. }
    19. const obj = {
    20.     a: 1
    21. }
    23. fn.bind(obj, 2, 3); // 6