4.join将数组拼接成字符串

var arr=["html","css","js"];
var b=arr.join();
var c=arr.join("");
var d=arr.join("/"); 
console.log(b);//html,css,js
console.log(c);//htmlcssjs
console.log(d); //html/css/js
console.log(typeof b);

例子
实现 1994/犯罪/剧情

 <p id="p"></p>
    <script>
        var p=document.getElementById("p")
        var arr=[1994,"犯罪","剧情"];  //1994/犯罪/剧情
        var str=arr.join("/");  
        p.innerHTML=str;
        console.log(str);
    </script>