call、apply、bind实现

1. call

// 将函数设为对象的属性 -> 执行该函数 -> 删除该函数 --> 返回函数执行返回值
obj.f = fn
let ret = obj.f()
delele obj.f
return ret

function call(fn, context, ...args) { 
  context = context || window // 默认为window
  context.f = fn
  let ret = context.f(...args)
  delete context.f
  return ret
}

2. apply

和call差不多，只是第三个参数是数组时忽略其后的参数，遍历传入即可

function apply(fn, context, ...args) {
  context = context || window
  context.f = fn
  if(Array.isArray(args[0])) args = args[0] // 多出这一行
  let ret = context.f(...args)
  delete context.f
  return ret
}

3. bind实现

通过闭包的方式，存储context和args，返回的函数命名为fBind

function bind(fn, context, ...args) {
  context = context || window
  return function fBind(...args2) {
    if (this instanceof fBind) return new fn(...args, ...args2)
    return fn.apply(context, [...args, ...args2])
  }
}