实现apply方法

Function.prototype.myApply = function (context, args = []) {
  let ctx = context || window
  // 将当前被调用的方法定义在ctx.func上，对象方法绑定this
  // 新建一个唯一的值存
  let func = Symbol()
  ctx[func] = this
  const res = args.length > 0 ? ctx[func](...args) : ctx[func]()
  delete ctx[func]
  return res
}

Function.prototype.myApply = function (context = window, ...args) {
  // this --> func   context --> obj args --> 传递过来的参数

  // 在context上加一个唯一值不影响context上的属性
  let key = Symbol('key')
  context[key] = this
  let result = context[key](args)
  delete context[key]
  return result
}

Function.prototype.myApply = function (context, args) {
  if (!context || context === null) {
    context = window
  }
  // 创建唯一的key
  let fn = Symbol()
  context[fn] = this
  return context[fn](...args)
}

Function.prototype.myApply = function (context, arr) {
  var context = Object(context) || window
  context.fn = this

  var result
  if (!arr) {
    result = context.fn()
  } else {
    var args = []
    for (var i = 0, len = arr.length; i < len; i++) {
      args.push('arr[' + i + ']')
    }
    result = eval('context.fn(' + args + ')')
  }
  delete context.fn
  return result
}