17. 实现函数原型方法

call

Function.prototype.call2 = function (context) {
  context = context || window;
  context.fn = this;
  const args = [...arguments].slice(1);
  const result = context.fn(...args);
  delete context.fn;
  return result;
};

apply

Function.prototype.apply2 = function (context, arr) {
  context = context || window;
  context.fn = this;
  let result;
  if (!arr) {
    result = context.fn();
  } else {
    result = context.fn(...arr);
  }
  delete context.fn;
  return result;
};

bind

Function.prototype.bind2 = function (context) {
  if (typeof this !== 'function') {
    throw new Error('Function.prototype.bind - what is trying to be bound is not callable');
  }
  const self = this;
  const args = [...arguments].slice(1);
  return function F() {
    if (this instanceof F) {
      return new self(...args, ...arguments);
    }
    return self.apply(context, args.concat(...arguments));
  };
};