近期的笔试面经

汇总一个笔试面经吧

有A、B、C三个任务，要求：A和B，C异步执行，A，B二者都完成后执行C

function promise1(){
setTimeout(
    ()=>{console.log(1)}),0
}
function promise2(){
setTimeout(
    ()=>{console.log(2)}),0
}
function promise3(){
setTimeout(
    ()=>{console.log(3)}),50
}
Promise.all([promise1(), promise2()]).then(promise3());

 给一个字符串找出其中的字母并将首字母大写后返回

 var str='123abcd234dae';
 function filters(str){
     var newStr=str.replace(/[0-9]/g,'');
     return newStr.charAt(0).toUpperCase()+newStr.substring(1);
 }
 console.log(filters(str));
 

 实现一个用hash实现一个路由跳转：

 <div id="index-page" class="content">
        <ul>
            <li><a href="#/1">1</a></li>
            <li><a href="#/2">2</a></li>
            <li><a href="#/3">3</a></li>
            <li><a href="#/4">4</a></li>
            <li><a href="#/5">5</a></li>
        </ul>
    </div>
    <div id="text-page" class="content">
        <h1>this is text page</h1>
        <a href="#/1">back</a>
    </div>
   <div id="news-page" class="content">
       <h1>this is new page</h1>
       <a href="#/1">back</a>
    </div>
    <div id="about-page" class="content">
        <h1>this is about page</h1>
       <a href="#/1">back</a> 
    </div>

   <script>
    function Router(){
        this.routes={};
        this.currentURL="";
    }
    Router.prototype.route=function(path,callback){
        this.routes[path]=callback||function(){};
    }
    Router.prototype.refresh=function(){
        this.currentURL=location.hash.slice(1)||'/1';
        try{
            this.routes[this.currentURL]();
        }
        catch(err){
            alert("请输入合法路由");
        }}
        //路由初始化，添加监听事件
        Router.prototype.init=function(){
            var that=this;
            if(window.addEventListener){
              window.addEventListener("load",function(){
                  that.refresh();
              },false);
        if("onhashchange" in window.document.body){
            window.addEventListener("hashchange",function(){
                that.refresh();
            },false)
        } 
        else{
            onhashchange_compatibility(that);
        }   }
        else if(window.attachEvent){
            window.attachEvent('onload',function(){that.refresh()},false);
            window.attachEvent("onhashchange",function(){that.refresh();},false)
            onhashchange_compatibility(that);
        }
        }
        function onhashchange_compatibility(that){
            var location=window.location;
            oldURL=location.href;
            oldHash=location.hash;
            setInterval(function(){
                var newHash=location.hash
                if(newHash!=oldHash){
                    that.refresh();
                    oldURL=location.href;
                    oldHash=newHash;
                }
            },100)
        }
function display_page(id){
    var el=document.getElementsByClassName("content");
    for(var i=0;i<el.length;i++){
        el[i].style.display="none";
    }
    el[id].style.display="block";
}
window.Router=new Router();
Router.route('/1',function(){
    display_page(0);
})
Router.route('/2',function(){
    display_page(1);
})
Router.route('/3',function(){
    display_page(2);
})
Router.route('/4',function(){
    display_page(3);
})
window.Router.init();
    </script>

 js实现一个map数据结构：

function Map(){
this.container=new Object();
}
Map.prototype.set=function(key, value)
{
if(key!=null&&key!='')
{
this.container[key]=value;
}
}
Map.prototype.get=function(key){
return this.container[key]
}
Map.prototype.has=function(key){
for(var i in this.container){
if(i===key)
return true;
}
}
Map.prototype.delete=function(key){
delete this.container
this.container=new Object();
}
Map.prototype.size=function(){
var count;
for(var i in container)
count++
return count;
}

 一道笔试题的知识点，题目如下：

var a={n:1}
var b=a
a.x=a={n:2} 
console.log(a.n,b.n)//2 1
console.log(a.x,b.x)//undefined Object

 首先分析3处，等号赋值是从右往左赋值

然后如图：