用js实现cookie的读、写、全部删除和删除指定cookie值的删除---源码

//设置cookie的值，以及过期时间
function setCookie(cname,cvalue,exdays){
    var d = new Date();
    d.setTime(d.getTime()+(exdays*24*60*60*1000));
    var expires = "expires="+d.toGMTString();
    document.cookie = cname+"="+cvalue+"; "+expires;
}
//得到cookie的值
function getCookie(cname){
    var name = cname + "=";
    var ca = document.cookie.split(';');
    for(var i=0; i<ca.length; i++) {
        var c = ca[i].trim();
        if (c.indexOf(name)==0) { return c.substring(name.length,c.length); }
    }
    return "";
}
//删除指定cookie的值
function removeCookie(cname){
    var name = cname + "=";
    var ca = document.cookie.split(';');    
    var cookieStr="";
    for(var i=0; i<ca.length; i++) {
        var c = ca[i].trim();
        if (c.indexOf(name)==0) {
            document.cookie =c + ';expires=' + new Date(0).toUTCString()  
        }else{
            cookieStr+=c;
            cookieStr+=";";
        }
        document.cookie =cookieStr;
    
    }
    
}
//删除全部的cookie值
function clearCookie(){ 
var keys=document.cookie.match(/[^ =;]+(?==)/g); 
if (keys) { 
for (var i = keys.length; i--;) ;
document.cookie=keys[i]+'=0;expires=' + new Date( 0).toUTCString() ;
} 
} 

//对cookie的使用
function checkCookie(){
    var user=getCookie("name");
    removeCookie("name");
    if (user!=""){
        alert("欢迎 " + user + " 再次访问");
    }
    
    else {
        user = prompt("请输入你的名字:","");
          if (user!="" && user!=null){
            setCookie("username",user,30);
        }
    }
}