常见前端面试代码题

//this绑定，立即执行函数表达式
var num = 1;
var myObject = {
    num:2,
    add:function(){
        this.num = 3;
        (function(){
            console.log(this.num);
            this.num = 4;
        })();
        console.log(this.num);
    },
    sub:function(){
        console.log(this.num);
    }
}
myObject.add();
console.log(myObject.num);
console.log(num);
var sub = myObject.sub;
sub();
//结果  1 3 3 4 4

var number = 5;
var obj = {
    number: 3,
    fn1: (function () {
        var number;
        this.number *= 2;
        number = number * 2;
        number = 3;
        return function () {
            var num = this.number;
            this.number *= 2;
            console.log(num);
            number *= 3;
            console.log(number);
        }
    })()
}
var fn1 = obj.fn1;
fn1.call(null);
obj.fn1();
console.log(window.number);
//结果  10 9 3 27 20

var name = ‘window‘
var person1 = {
    name:‘person1‘,
    show1:function(){
        console.log(this.name);
    },
    show2:()=>console.log(this.name),
    show3:function(){
        return function(){
            console.log(this.name);
        }
    },
    show4:function(){
        return ()=>console.log(this.name)
    }
}
var person2 = {name:‘person2‘};
person1.show1();//person1
person1.show1.call(person2);//person2

person1.show2();//window
person1.show2.call(person2);//window

person1.show3()();//person1错    window
person1.show3().call(person2);//person2
person1.show3.call(person2)();//person2错   window

person1.show4()();//window错     person1
person1.show4().call(person2);//person1
person1.show4.call(person2)();//window错    person2

var name = "hanmeimei";
var f = function(){
    var name = "lilei";
    return function(){
        console.log(this.name);
    } 
}

f()();//hanmeimei

function getRandom(num){
    var numb = [];
    var newArr = [];
    while(numb.length<num){
        let n = parseInt(Math.random() * 32);
        if(n>=2&&n<=32){
            numb.push(n);
            newArr = Array.from(new Set(numb));
        } 
    }
    console.log(newArr);
    //return newArr;
}
getRandom(10);

// 写出一个数组展开函数, 如输入：[1,[2,[3,4,2],2],5,[6]],  则输出：[1,2,3,4,2,2,5,6] 
// 因为和深度无关，所以说最简单可以这样
function flatten(arr){
    var res = arr.join().split(‘,‘);
    res = res.map( ele => +ele)
    return res;
}
// 还有吗，递归，写一下
function flatten(arr){
    var array = [];
    arr.forEach(ele => {
        if(Array.isArray(ele)){
            array.push(...flatten(ele));
        } else {
            array.push(ele);
        }
    })
    return array;
}
var arr1 = [1,[2,[3,4,2],2],5,[6]];
flatten(arr1)

console.log(‘begin‘);
setTimeout(() => {
    console.log(‘setTimeout 1‘);
    Promise.resolve().then(() => {
        console.log(‘promise 1‘);
        setTimeout(() => {
            console.log(‘setTimeout2 between promise1&2‘);
        })
    }).then(() => {
        console.log(‘promise 2‘);
    });
}, 0);
console.log(‘end‘);

begin
end
setTimeout 1
promise 1
promise 2
setTimeout2 between promise1&2

console.log(‘script start‘)

async function async1() {
  await async2()
  console.log(‘async1 end‘)
}
async function async2() {
  console.log(‘async2 end‘) 
}
async1()

setTimeout(function() {
  console.log(‘setTimeout‘)
}, 0)

new Promise(resolve => {
  console.log(‘Promise‘)
  resolve()
})
  .then(function() {
    console.log(‘promise1‘)
  })
  .then(function() {
    console.log(‘promise2‘)
  })

console.log(‘script end‘)
//结果
script start
end
Promise
end
end
promise1
promise2
undefined
setTimeout

console.log(Number([1]))
if([]==false){console.log(1)}//1
if({}==false){console.log(2)}
if([]){console.log(3)}//3
if([1]==[1]){console.log(4)}

function Fibonacci(n)
{
  if (n == 0) {
    return 1;
  }
  var pre = 1, // 前一个值
    cur = 1; // 当前值
  for (var i = 2; i <= n; i++) { // 更新两个值
    cur += pre;
    pre = cur - pre;
  }
  return cur;
}

function Fibonacci2(n)
{
    var f = [0, 1];
    for(let i = 2; i <= n; i++) {
        f[i] = f[i - 1] + f[i - 2];
    }
    return f[n];
}

Fibonacci(5)

var getUrlParam2 = function(sUrl, sKey){
    // 捕获组两枚,一枚负责Key 一枚负责获取Value
    var reg = new RegExp(‘([0-9a-zA-Z%]+)=([0-9a-zA-Z%]+)&*‘, ‘ig‘);
    // 结果集保存
    var result = {};
    var temp;
    var key, value;
    while(temp = reg.exec(sUrl)) {
        key = temp[1];
        value = temp[2];
        if (result[key] && !(result[key] instanceof Array)) {
            result[key] = [result[key], value];     // 如果发现了第二个同名参数,则从字符串变为数组
        } else if (result[key] instanceof Array) {
            result[key].push(value);    // 已经是数组,又找到了,就push进去
        } else {
            result[key] = value;    // 第一次,还是普通保存
        }
    }
    if (sKey) {
        return result[sKey] ? result[sKey] : ‘‘;    // 为了避免undefined的情况
    } else {
        return result;  // 返回全部的对象参数
    }
 
}
getUrlParam2("http://www.nowcoder.com?key=1&key=2&key=3&test=4#hehe")

//http://www.baidu.com?aa=11&callback=my_jsonp04349289664328899
    var jsonp = function(url,param,callback){
        //处理url地址,查找？，如果没有？这个变量就有一个"?"，有？这个变量接收一个&
        var querystring = url.indexOf("?") == -1?"?":"&";

        //处理参数{xx:xx}
        for(var k in param) {
            querystring += k + "=" + param[k] + ‘&‘;//?k=para[k]
        }
        //处理回调函数名
        var random = Math.random().toString().replace(".","");
        var cbval = "my_jsonp" + random;
        var cb = "callback="+cbval;
        querystring += cb;
        var script = document.createElement("script");
        script.src = url + querystring;
        //把回调函数的名字附给window
        window[cbval] = function(param) {
            //这里执行回调的操作，用回调来处理参数
          callback(param);
          //拿到了就删掉这个script
          document.body.removeChild(script);
        };
        document.body.appendChild(script);
    }
    jsonp(
        "https://www.baidu.com",
        {aa:11},
        function(){
            console.log(param);
        }
        );

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script type="text/javascript">
    // 得到航班信息查询结果后的回调函数
    var flightHandler = function(data){
        alert(‘你查询的航班结果是：票价 ‘ + data.price + ‘ 元，‘ + ‘余票 ‘ + data.tickets + ‘ 张。‘);
    };
    // 提供jsonp服务的url地址（不管是什么类型的地址，最终生成的返回值都是一段javascript代码）
    var url = "http://flightQuery.com/jsonp/flightResult.aspx?code=CA1998&callback=flightHandler";
    // 创建script标签，设置其属性
    var script = document.createElement(‘script‘);
    script.setAttribute(‘src‘, url);
    // 把script标签加入head，此时调用开始
    document.getElementsByTagName(‘head‘)[0].appendChild(script); 
    </script>
</head>
<body>
</body>
</html>

//js的深克隆
function deepCopy(obj){
    //判断是否是简单数据类型，
    if(typeof obj == "object"){
        //复杂数据类型
        var result = obj.constructor == Array ? [] : {};
        for(let i in obj){
            result[i] = typeof obj[i] == "object" ? deepCopy(obj[i]) : obj[i];
        }
    }else {
        //简单数据类型 直接 == 赋值
        var result = obj;
    }
    return result;
}

//判断是否为数组  通过toString()方法  通过constructor 通过instanceof  Array.isArray
var arr = [1, 2, 3];
        var obj = {
            name: ‘lyl‘,
            age: 18,
            1: ‘name‘
        }
        console.log(Object.prototype.toString.call(arr) === ‘[object Array]‘); //true
        console.log(Object.prototype.toString.call(boj) === ‘[object Array]‘); //false

// 这里我们创建了一个构造函数 参数就是执行器
function Promise(exector) {
    // 这里我们将value 成功时候的值 reason失败时候的值放入属性中
    let self = this;
    // 这里我们加入一个状态标识
    this.status = ‘pending‘;
    this.value = undefined;
    this.reason = undefined;
    // 成功执行
    function resolve(value) {
        // 判断是否处于pending状态
        if (self.status === ‘pending‘) {
            self.value = value;
            // 这里我们执行之后需要更改状态
            self.status = ‘resolved‘;
        }
    }   
    // 失败执行
    function reject(reason) {
        // 判断是否处于pending状态
        if (self.status === ‘pending‘) {
            self.reason = reason;
            // 这里我们执行之后需要更改状态
            self.status = ‘rejected‘;
        }
    }  
    // 这里对异常进行处理
     try {
        exector(resolve, reject);
    } catch(e) {
        reject(e)
    }
}
// 我们将then方法添加到构造函数的原型上 参数分别为成功和失败的回调
Promise.prototype.then = function(onFulfilled, onRejected) {
    // 获取下this
    let self = this;
    if (this.status === ‘resolved‘) {
        onFulfilled(self.value);
    }
    if (this.status === ‘rejected‘) {
        onRejected(self.reason);
    }
}
//运行
let promise = new Promise((resolve, reject) => {
     resolve("haha");
})
promise.then(data => {
    console.log(data); //输出 haha
}, err=> {
    console.log(err);
})
// 多次调用
promise.then(data => {
    console.log(data); //输出 haha
}, err=> {
    console.log(err);
})

// 当出现连续数字的时候以‘-’输出  [1, 2, 3, 4, 6, 8, 9, 10]
// 期望结果  ["1-4", 6, "8-10"]
var arrange = function(arr){
    var result = [],temp = [];
    arr.sort(function(source, dest){
        return source - dest;
    }).concat(Infinity).reduce(function(source, dest){
        temp.push(source);
        if(dest-source > 1){
            result.push(temp);
            temp = [];
        }
        return dest;
    });
    return result;
};
var formatarr = function(arr) {
    var newArr = []
    var arr1 = arrange(arr)
    for (var i in arr1) {
        var str = ‘‘;
        if (arr1[i].length > 1) {
            str = arr1[i][0] + ‘-‘ + arr1[i][arr1[i].length - 1];
            newArr.push(str)
        } else {
            newArr.push(arr1[i][0]);
        }
   }
   return newArr;
}

function revStr(str){
    var temStr = "";
    var len = str.length;
    for(var i=len-1;i>=0;i--){
        temStr = str.charAt(i);
    }
    return temStr;
}
//字符串的完全翻转
str.split("").reverse().join("")
//翻转字符串中单词顺序,但单词字母顺序不变:
function reverseStr(param){
    var arr = param.split(" ");
    var newArr = [];
    for(i=0;i<arr.length;i++){
        newArr[arr.length-i] = arr[i];
    }
    return newArr.join(" ");
}
document.write(reverseStr("smile at life"));        

var str = "hello world";
function isString(str){
    if(typeof str == "string" || str.constructor == String){
        return true;
    }else{
        return false;
    }
}
if(typeof Array.isArray===‘undefined‘){ 
　　Array.isArray=function(arg){ 
　　　　return Object.prototype.tostring.call(rag)===‘[Object Array]‘; 
　　}; 
}

function findNullOfNum(arr1,arr2){
    var str = arr2.split("");
    var result = [];
    for(var i=0,x=0;i<arr1.length;i++){
        if(str.indexOf(arr1[i] == -1){
           result[x] = arr1[i];
           x++;
        }
     }
    return result;
}

function select(arr){
  var temp=[];
  for(var i = 0;i < arr.length;i++){
    //indexOf() 方法可返回某个指定的字符串值在字符串中首次出现的位置。ie8不兼容
    if(temp.indexOf(arr[i]) == -1){
      temp.push(arr[i]);
    }
  }
  return temp;
}
//具体验证
var arr=[4,5,1,53,5,1,3];
console.log(select(arr));

//includes():判断当前数组是否包含某个值，若有则返回true,若没有则返回false;
function select(arr){
  var temp=[];
  arr.forEach((v)=>{
    temp.includes(v)||temp.push(v);
  })
  return temp;
}
//循环判断
function select(arr){
  var temp=[];
  for(var i = 0; i < arr.length; i++){
    for(var j = i+1; j < arr.length; j++){
      if(arr[i] === arr[j]){
        j = ++i;
      }
    }
    temp.push(arr[i]);
  }
  return temp;
}