js去空格函数

分别去字符串前后，左边，右边空格

  String.prototype.trim = function(){ return this.replace(/^\s+|\s+$/g,"")}
  String.prototype.ltrim = function(){ return this.replace(/^\s+/g,"")}
  String.prototype.rtrim = function(){ return this.replace(/\s+$/g,"")}