JS 实现 startWith endWith函数

 String.prototype.startWith = function(s) { 
    if (s == null || s == "" || this.length == 0 || s.length > this.length) 
        return false; 
if (this.substr(0, s.length) == s) 
return true;
else 
return false; 
    return true; 
}

String.prototype.endWith = function(s) {
     if (s == null || s == "" || this.length == 0|| s.length > this.length)
          return false;
     if (this.substring(this.length - s.length) == s)
          return true;
     else
          return false;
    return true;
}