/*
* Esto funciona para:
* function variableFunction(requiredParam, variableParam, callback) {...}
* function variableFunction(requiredParam, variableParam) {...}
* function variableFunction(requiredParam, callback) {...}
*/

function variableFunction(requiredParam, variableParam, callback){
   if(!requiredParam) throw new Error("Required Param");
   if(!callback && typeof variableParam === 'function'){
      callback = variableParam;
      variableParam = undefined;
   }
// ... add your code.
}

/*
* Esto funciona para:
* function variableFunction(requiredParam, variableParam, callback) {...}
* function variableFunction(requiredParam, variableParam) {...}
* function variableFunction(requiredParam, callback) {...}
*/
function variableFunction(requiredParam, variableParam, callback = variableParam){
   if(!requiredParam) throw new Error("Required Param");
   if(typeof variableParam === 'function'){
      variableParam = undefined;
   }

// ... add your code.
}