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;

}