【call apply bind】源码实现

call方法的实现

  Function.prototype.myCall = function (context) {
      //this 调用的函数
      //context 调用函数的对像

      // 判断this是否为函数
      if (typeof this !== 'function') {
        throw TypeError('Error')
      }
      context = context || window
      //声明一个Symbol 防止fn被占用
      const fn = Symbol('fn')
      //将函数添加成为contxt 对象的方法被调用
      context[fn] = this
      //获取参数
      //const args = Array.prototype.slice.call(arguments, 1)
      //const args = Array.from(arguments).slice(1)
      const args = [...arguments].slice(1)
      //调用返回结果
      const result = context[fn](...args)
      //删除方法
      delete context[fn]
      //返回结果
      return result
  }

apply方法实现

  Function.prototype.myApply = function (context){
    if (typeof this !== 'function') {
      throw TypeError('Error')
    }

    context = context || window

    const args = arguments[1]

    const fn = Symbol('fn')

    context[fn] = this

    const result = context[fn](...args)

    delete context[fn]

    return result

  }

bind方法实现

  Function.prototype.myBind = function (context){
    if (typeof this !== 'function') {
      throw TypeError('Error')
    }
    const args = [...arguments].slice(1)
    let _this = this

    return function F(){
      if (this instanceof F) {
        return new _this(...args, ...arguments)
      } else {
        return _this.call(context, ...args, ...arguments)
      }
    }
  }

  //例题
  function print(age) {
    console.log(this.name+ "" + age)
  }

  let obj = {
    name: "李华"
  }

  print.myCall(obj,1,2,3) //李华1
  print.myApply(obj,[3,2,1]) //李华3

new方法实现

  function myNew(){
    //创建一个新对象
    let obj = new Object()
    //获取到构造函数
    let Constructor = Array.prototype.shift.call(arguments)
    //将对象的__proto__(隐形原型属性) == 构造函数的 prototype 显示原型属性
    obj.__proto__ = Constructor.prototype
    //将构造函数作为对象的方法被调用
    let ret = Constructor.apply(obj, arguments)
    // 如果构造函数返回值是对象则返回这个对象，如果不是对象则返回新的实例对象
    return typeof ret === "object" && ret !== null ? ret : obj
  }

  //例题：
  // ========= 无返回值 =============
  // const testNewFun = function(name) {
  //   this.name = name;
  // };
  //
  // const newObj = myNew(testNewFun, 'foo');
  //
  // console.log(newObj); // { name: "foo" }
  // console.log(newObj instanceof testNewFun); // true

  // ========= 有返回值 =============
  const testNewFun = function(name) {
    this.name = name;
    return {};
  };

  const newObj = myNew(testNewFun, 'foo');

  console.log(newObj); // {}
  console.log(newObj instanceof testNewFun); // false

reduce实现

  Array.prototype.myReduce = function (callbackFn){
    let o = this
    let k = 0,
        kPresent = false,
        accumulator,
        initialValue = arguments.length > 0 ? arguments[1] : undefined,
        len = this.length

    if (typeof callbackFn !== 'function') {
        throw TypeError('Error')
    }

    if (len === 0 && initialValue !== undefined) {
        throw TypeError('Error')
    }

    if (initialValue !== undefined) {
        accumulator = initialValue
    } else {
        accumulator = o[k]
        ++k
    }

    while (k < len) {
        kPresent = o.hasOwnProperty(k)
        if (kPresent) {
            accumulator = callbackFn.apply(undefined, [accumulator, o[k], k, o])
        }
        ++k
    }

    return accumulator
  }

  const rReduce = ['1', null, undefined, , 3, 4].reduce((a, b) => a + b, 3);
  const mReduce = ['1', null, undefined, , 3, 4].myReduce((a, b) => a + b, 3);

  console.log(rReduce, mReduce)