JavaScript-join连接符

 1.转字符串:2种
    1.将数组中每个元素都转为字符串,再用逗号分隔:var str=String(arr);
    2.将数组中每个元素都字符串，再用自定义下标连接每个元素
     var str=arr.join("自定义连接符")
    //无缝拼接
    var chars=["H","e","l","l","o"];
    console.log(chars.join(""));
    //单词拼接为句子
    var words=["no","zuo","no","die"];
    console.log(words.join(" "));//"空格"
    //判断是否为空数组
    var arr=[,,,,,,,,];
    console.log(arr);
    //动态生成页面元素
    var provs=["北京","南京","上海"];
    var html="<option>"+provs.join("</option>")+"</option>";
    //设置id为sel的元素开始标签到结束标签之间的内容为html
    sel.innerHTML="<option>请选择</option>"