2022 秋招前端面试题（四）（附答案）

async function async1() {  console.log("async1 start");  await async2();  console.log("async1 end");}async function async2() {  console.log("async2");}async1();console.log('start')复制代码

async1 startasync2startasync1 end复制代码

Array.prototype.forEach1 = function(callback, thisArg) {    if(this == null) {        throw new TypeError('this is null or not defined');    }    if(typeof callback !== "function") {        throw new TypeError(callback + 'is not a function');    }    // 创建一个新的 Object 对象。该对象将会包裹(wrapper)传入的参数 this(当前数组)。    const O = Object(this);    // O.length >>> 0 无符号右移 0 位    // 意义：为了保证转换后的值为正整数。    // 其实底层做了 2 层转换，第一是非 number 转成 number 类型，第二是将 number 转成 Uint32 类型    const len = O.length >>> 0;    let k = 0;    while(k < len) {        if(k in O) {            callback.call(thisArg, O[k], k, O);        }        k++;    }}复制代码

Array.prototype.map1 = function(callback, thisArg) {    if(this == null) {        throw new TypeError('this is null or not defined');    }    if(typeof callback !== "function") {        throw new TypeError(callback + 'is not a function');    }    const O = Object(this);     const len = O.length >>> 0;    let newArr = [];  // 返回的新数组    let k = 0;    while(k < len) {        if(k in O) {            newArr[k] = callback.call(thisArg, O[k], k, O);        }        k++;    }    return newArr;}复制代码

Array.prototype.filter1 = function(callback, thisArg) {    if(this == null) {        throw new TypeError('this is null or not defined');    }    if(typeof callback !== "function") {        throw new TypeError(callback + 'is not a function');    }    const O = Object(this);     const len = O.length >>> 0;    let newArr = [];  // 返回的新数组    let k = 0;    while(k < len) {        if(k in O) {            if(callback.call(thisArg, O[k], k, O)) {                newArr.push(O[k]);            }        }        k++;    }    return newArr;}复制代码

Array.prototype.some1 = function(callback, thisArg) {    if(this == null) {        throw new TypeError('this is null or not defined');    }    if(typeof callback !== "function") {        throw new TypeError(callback + 'is not a function');    }    const O = Object(this);     const len = O.length >>> 0;    let k = 0;    while(k < len) {        if(k in O) {            if(callback.call(thisArg, O[k], k, O)) {                return true            }        }        k++;    }    return false;}复制代码

Array.prototype.reduce1 = function(callback, initialValue) {    if(this == null) {        throw new TypeError('this is null or not defined');    }    if(typeof callback !== "function") {        throw new TypeError(callback + 'is not a function');    }    const O = Object(this);    const len = O.length >>> 0;    let k = 0;    let accumulator = initialValue;    // 如果第二个参数为undefined的情况下，则数组的第一个有效值（非empty）作为累加器的初始值    if(accumulator === undefined) {        while(k < len && !(k in O)) {            k++;        }        // 如果超出数组界限还没有找到累加器的初始值，则TypeError        if(k >= len) {            throw new TypeError('Reduce of empty array with no initial value');        }        accumulator = O[k++];    }    while(k < len) {        if(k in O) {            accumulator = callback(accumulator, O[k], k, O);        }        k++;    }    return accumulator;}复制代码

function shallowClone(obj) {  let cloneObj = {};
  for (let i in obj) {    cloneObj[i] = obj[i];  }
  return cloneObj;}复制代码

function deepCopy(obj) {  if (typeof obj === 'object') {    var result = obj.constructor === Array ? [] : {};
    for (var i in obj) {      result[i] = typeof obj[i] === 'object' ? deepCopy(obj[i]) : obj[i];    }  } else {    var result = obj;  }
  return result;}复制代码

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。复制代码

function unique(arr) {    var res = arr.filter(function(item, index, array) {        return array.indexOf(item) === index    })    return res}复制代码

var unique = arr => [...new Set(arr)]复制代码

function debounce(func, wait) {    var timeout;    return function () {        var context = this;        var args = arguments;        clearTimeout(timeout)        timeout = setTimeout(function(){            func.apply(context, args)        }, wait);    }}复制代码

var node = document.getElementById('layout')function getUserAction(e) {    console.log(this, e)  // 分别打印：node 这个节点 和 MouseEvent    node.innerHTML = count++;};node.onmousemove = debounce(getUserAction, 1000)复制代码

function debounce(func, wait, immediate) {    var timeout, result;
    var debounced = function () {        var context = this;        var args = arguments;
        if (timeout) clearTimeout(timeout);        if (immediate) {            // 如果已经执行过，不再执行            var callNow = !timeout;            timeout = setTimeout(function(){                timeout = null;            }, wait)            if (callNow) result = func.apply(context, args)        } else {            timeout = setTimeout(function(){                func.apply(context, args)            }, wait);        }        return result;    };
    debounced.cancel = function() {        clearTimeout(timeout);        timeout = null;    };
    return debounced;}复制代码

var setUseAction = debounce(getUserAction, 10000, true);// 使用防抖node.onmousemove = setUseAction
// 取消防抖setUseAction.cancel()复制代码

function currying(fn, ...args) {  const length = fn.length;  let allArgs = [...args];  const res = (...newArgs) => {    allArgs = [...allArgs, ...newArgs];    if (allArgs.length === length) {      return fn(...allArgs);    } else {      return res;    }  };  return res;}
// 用法如下：// const add = (a, b, c) => a + b + c;// const a = currying(add, 1);// console.log(a(2,3))复制代码