String.prototype.realLength = function() {
    return this.replace(/[^\x00-\xff]/g, "**").length; // [^\x00-\xff] - 匹配非双字节的字符 
};
String.prototype.realSubstring = function(n){
    var str = this||‘‘;
    if(this.realLength()<=n){return this;}
    var m=Math.floor(n/2);
    for(var i=m;i<this.length;i++){
        if(this.substr(0,i).realLength()>=n){
            return this.substr(0,i)+"...";
        }
    }
    return this;
}