call,apply,bind简单实现

const log = console.log.bind(this)

const obj = {
  val: '这是call'
}
function fn(B, X) {
  return {
    b: B,
    a: this.val,
    x: X
  }
}

call

Function.prototype.calll = function (target) {
  const fn = Symbol('fn')
  target[fn] = this
  const args = [...arguments].slice(1)
  const ret = target[fn](...args)
  delete target[fn]
  return ret
}

const res1 = fn.calll(obj, '这是B')
log('res1', res1)  //{b: '这是B', a: '这是call', x: undefined}

apply

Function.prototype.applyy = function (target) {
  const fn = Symbol('fn')
  target[fn] = this
  const args = [...arguments]?.[1]
  const ret = target[fn](...args)
  delete target[fn]
  return ret
}


const res2 = fn.applyy(obj, ['这是B'])
log('res2', res2)  //{b: '这是B', a: '这是call', x: undefined}

bind

Function.prototype.bindd = function (target) {
  const self = this
  const args = Array.prototype.slice.call(arguments, 1)
  function func() {
    const newArgs = Array.prototype.slice.call(arguments)
    return self.apply(this instanceof func ? func : target, args.concat(newArgs))
  }
  func.prototype = Object.create(this.prototype) // new出来的实例可以继承绑定函数的方法
  return func
}

const Res = fn.bindd(obj, '这是B')
const ret = new Res('这是X')
log('call', ret) //{b: '这是B', a: undefined, x: '这是X'}