Javascript String toUnderscoreCustom()

String.prototype.toUnderscoreCustom = function(){
 var primera_letra_minuscula =  this.substring(0, 1).toLowerCase();
 var el_resto = this.substring(1, this.length);
 var _this = primera_letra_minuscula + el_resto;
 return _this.replace(/([A-Z])/g, function($1){return "_"+$1.toLowerCase();});
};