2023前端面试知识点总结_2023-02-24

JavaScript中的对象都有一个特殊的 prototype 内置属性，其实就是对其他对象的引用
几乎所有的对象在创建时 prototype 属性都会被赋予一个非空的值，我们可以把这个属性当作一个备用的仓库
当试图引用对象的属性时会出发get操作，第一步时检查对象本身是否有这个属性，如果有就使用它，没有就去原型中查找。一层层向上直到Object.prototype顶层

基于原型扩展描述一下原型链，什么是原型链，原型的继承，ES5和ES6继承与不同点。

Promise.resolve(1)
  .then(2)
  .then(Promise.resolve(3))
  .then(console.log)

1

var a, b
(function () {
   console.log(a);
   console.log(b);
   var a = (b = 3);
   console.log(a);
   console.log(b);   
})()
console.log(a);
console.log(b);

undefined 
undefined 
3 
3 
undefined 
3

function foo(something){
    this.a = something
}

var obj1 = {
    foo: foo
}

var obj2 = {}

obj1.foo(2); 
console.log(obj1.a); // 2

obj1.foo.call(obj2, 3);
console.log(obj2.a); // 3

var bar = new obj1.foo(4)
console.log(obj1.a); // 2
console.log(bar.a); // 4

Promise.resolve = function(value) {
    // 1.如果 value 参数是一个 Promise 对象，则原封不动返回该对象
    if(value instanceof Promise) return value;
    // 2.如果 value 参数是一个具有 then 方法的对象，则将这个对象转为 Promise 对象，并立即执行它的then方法
    if(typeof value === "object" && 'then' in value) {
        return new Promise((resolve, reject) => {
           value.then(resolve, reject);
        });
    }
    // 3.否则返回一个新的 Promise 对象，状态为 fulfilled
    return new Promise(resolve => resolve(value));
}

let template = '我是{{name}}，年龄{{age}}，性别{{sex}}';
let data = {
  name: '姓名',
  age: 18
}
render(template, data); // 我是姓名，年龄18，性别undefined

function render(template, data) {
  let computed = template.replace(/\{\{(\w+)\}\}/g, function (match, key) {
    return data[key];
  });
  return computed;
}

class Promise{
  constructor(executor) {
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    let resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
      }
    };
    let reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
      }
    };
    try {
      // 立即执行函数
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }
  then(onFulfilled, onRejected) {
    if (this.state === 'fulfilled') {
      let x = onFulfilled(this.value);
    };
    if (this.state === 'rejected') {
      let x = onRejected(this.reason);
    };
  }
}

function myPromise(constructor){ let self=this;
  self.status="pending" //定义状态改变前的初始状态 
  self.value=undefined;//定义状态为resolved的时候的状态 
  self.reason=undefined;//定义状态为rejected的时候的状态 
  function resolve(value){
    //两个==="pending"，保证了了状态的改变是不不可逆的 
    if(self.status==="pending"){
      self.value=value;
      self.status="resolved"; 
    }
  }
  function reject(reason){
     //两个==="pending"，保证了了状态的改变是不不可逆的
     if(self.status==="pending"){
        self.reason=reason;
        self.status="rejected"; 
      }
  }
  //捕获构造异常 
  try{
      constructor(resolve,reject);
  }catch(e){
    reject(e);
    } 
}
myPromise.prototype.then=function(onFullfilled,onRejected){ 
  let self=this;
  switch(self.status){
    case "resolved": onFullfilled(self.value); break;
    case "rejected": onRejected(self.reason); break;
    default: 
  }
}

// 测试
var p=new myPromise(function(resolve,reject){resolve(1)}); 
p.then(function(x){console.log(x)})
//输出1

const PENDING = "pending"; 
const FULFILLED = "fulfilled"; 
const REJECTED = "rejected";
const resolvePromise = (promise, x, resolve, reject) => {
  if (x === promise) {
    // If promise and x refer to the same object, reject promise with a TypeError as the reason.
    reject(new TypeError('循环引用'))
  }
  // if x is an object or function,
  if (x !== null && typeof x === 'object' || typeof x === 'function') {
    // If both resolvePromise and rejectPromise are called, or multiple calls to the same argument are made, the first call takes precedence, and any further calls are ignored.
    let called
    try { // If retrieving the property x.then results in a thrown exception e, reject promise with e as the reason.
      let then = x.then // Let then be x.then
      // If then is a function, call it with x as this
      if (typeof then === 'function') {
        // If/when resolvePromise is called with a value y, run [[Resolve]](promise, y)
        // If/when rejectPromise is called with a reason r, reject promise with r.
        then.call(x, y => {
          if (called) return
          called = true
          resolvePromise(promise, y, resolve, reject)
        }, r => {
          if (called) return
          called = true
          reject(r)
        })
      } else {
        // If then is not a function, fulfill promise with x.
        resolve(x)
      }
    } catch (e) {
      if (called) return
      called = true
      reject(e)
    }
  } else {
    // If x is not an object or function, fulfill promise with x
    resolve(x)
  }
}
function Promise(excutor) {
  let that = this; // 缓存当前promise实例例对象
  that.status = PENDING; // 初始状态
  that.value = undefined; // fulfilled状态时 返回的信息
  that.reason = undefined; // rejected状态时 拒绝的原因 
  that.onFulfilledCallbacks = []; // 存储fulfilled状态对应的onFulfilled函数
  that.onRejectedCallbacks = []; // 存储rejected状态对应的onRejected函数
  function resolve(value) { // value成功态时接收的终值
    if(value instanceof Promise) {
      return value.then(resolve, reject);
    }
    // 实践中要确保 onFulfilled 和 onRejected ⽅方法异步执⾏行行，且应该在 then ⽅方法被调⽤用的那⼀一轮事件循环之后的新执⾏行行栈中执⾏行行。
    setTimeout(() => {
      // 调⽤用resolve 回调对应onFulfilled函数
      if (that.status === PENDING) {
        // 只能由pending状态 => fulfilled状态 (避免调⽤用多次resolve reject)
        that.status = FULFILLED;
        that.value = value;
        that.onFulfilledCallbacks.forEach(cb => cb(that.value));
      }
    });
  }
  function reject(reason) { // reason失败态时接收的拒因
    setTimeout(() => {
      // 调⽤用reject 回调对应onRejected函数
      if (that.status === PENDING) {
        // 只能由pending状态 => rejected状态 (避免调⽤用多次resolve reject)
        that.status = REJECTED;
        that.reason = reason;
        that.onRejectedCallbacks.forEach(cb => cb(that.reason));
      }
    });
  }

  // 捕获在excutor执⾏行行器器中抛出的异常
  // new Promise((resolve, reject) => {
  //     throw new Error('error in excutor')
  // })
  try {
    excutor(resolve, reject);
  } catch (e) {
    reject(e);
  }
}
Promise.prototype.then = function(onFulfilled, onRejected) {
  const that = this;
  let newPromise;
  // 处理理参数默认值 保证参数后续能够继续执⾏行行
  onFulfilled = typeof onFulfilled === "function" ? onFulfilled : value => value;
  onRejected = typeof onRejected === "function" ? onRejected : reason => {
    throw reason;
  };
  if (that.status === FULFILLED) { // 成功态
    return newPromise = new Promise((resolve, reject) => {
      setTimeout(() => {
        try{
          let x = onFulfilled(that.value);
          resolvePromise(newPromise, x, resolve, reject); //新的promise resolve 上⼀一个onFulfilled的返回值
        } catch(e) {
          reject(e); // 捕获前⾯面onFulfilled中抛出的异常then(onFulfilled, onRejected);
        }
      });
    })
  }
  if (that.status === REJECTED) { // 失败态
    return newPromise = new Promise((resolve, reject) => {
      setTimeout(() => {
        try {
          let x = onRejected(that.reason);
          resolvePromise(newPromise, x, resolve, reject);
        } catch(e) {
          reject(e);
        }
      });
    });
  }
  if (that.status === PENDING) { // 等待态
// 当异步调⽤用resolve/rejected时 将onFulfilled/onRejected收集暂存到集合中
    return newPromise = new Promise((resolve, reject) => {
      that.onFulfilledCallbacks.push((value) => {
        try {
          let x = onFulfilled(value);
          resolvePromise(newPromise, x, resolve, reject);
        } catch(e) {
          reject(e);
        }
      });
      that.onRejectedCallbacks.push((reason) => {
        try {
          let x = onRejected(reason);
          resolvePromise(newPromise, x, resolve, reject);
        } catch(e) {
          reject(e);
        }
      });
    });
  }
};

foreach()方法会针对每一个元素执行提供得函数,该方法没有返回值,是否会改变原数组取决与数组元素的类型是基本类型还是引用类型
map()方法不会改变原数组的值,返回一个新数组,新数组中的值为原数组调用函数处理之后的值：

Promise.all = function(promises) {
    return new Promise((resolve, reject) => {
        if(Array.isArray(promises)) {
            if(promises.length === 0) return resolve(promises);
            let result = [];
            let count = 0;
            promises.forEach((item, index) => {
                Promise.resolve(item).then(
                    value => {
                        count++;
                        result[index] = value;
                        if(count === promises.length) resolve(result);
                    }, 
                    reason => reject(reason)
                );
            })
        }
        else return reject(new TypeError("Argument is not iterable"));
    });
}

//vue中购物车逻辑的实现
1. 购物车信息用一个数组来存储，数组中保存对象，对象中有id和count属性

2. 在vuex中state中添加一个数据 cartList 用来保存这个数组

3. 由于商品详情页需要用到加入购物车功能，所以我们需要提供一个mutation, 用来将购物车信息加入 cartList中

4. 加入购物车信息的时候，遵照如下规则： 如果购物车中已经有了该商品信息，则数量累加，如果没有该商品信息，则新增一个对象

5. 在商品详情页，点击加入购物车按钮的时候，调用vuex提供的addToCart这个mutation将当前的商品信息 （id count）传给addTocart  this.$store.commit("addToCart", {id:  , count：})

// js中购物车逻辑的实现
1.商品页点击“加入购物车”按钮，触发事件

2.事件调用购物车“增加商品”的Js程序（函数、对象方法）

3.向Js程序传递传递“商品id”、“商品数量”等数据

4.存储“商品id”、“商品数量”到浏览器的localStorage中

**展示购物车中的商品******

1.打开购物车页面

2.从localStorage中取出“商品Id”、“商品数量”等信息。

3.调用服务器端“获得商品详情”的接口得到购物车中的商品信息（参数为商品Id）

4.将获得的商品信息显示在购物车页面。

**完成购物车中商品的购买******

1.用户对购物车中的商品完成购买流程，产生购物订单

2.清除localStorage中存储的已经购买的商品信息

备注1：购物车中商品存储的数据除了“商品id”、“商品数量”之外，根据产品要求还可以有其他的信息，例如完整的商品详情（这样就不用掉服务器接口获得详情了）、购物车商品的过期时间，超过时间的购物车商品在下次打开网站或者购物车页面时被清除。

备注2：购物车商品除了存储在localStorage中，根据产品的需求不同，也可以存储在sessionStorage、cookie、session中，或者直接向服务器接口发起请求存储在服务器上。何种情况使用哪种方式存储、有啥区别请自己分析。

Promise.resolve(1)
  .then(2)
  .then(Promise.resolve(3))
  .then(console.log)

1
Promise {<fulfilled>: undefined}

Array.prototype.slice.call(arrayLike);

Array.prototype.splice.call(arrayLike, 0);

Array.prototype.concat.apply([], arrayLike);

Array.from(arrayLike);

当一个内部函数被调用，就会形成闭包，闭包就是能够读取其他函数内部变量的函数。
闭包作用：
局部变量无法共享和长久的保存，而全局变量可能造成变量污染，所以我们希望有一种机制既可以长久的保存变量又不会造成全局污染。