An example of params.

usingParams.js

//         1        2      2.1       2.2    2.3  2.4
app.param("id", function(request, response, next, id){
   // we check our id value using a function
   if(myCheckerFunction(id)){
      // we call next to continuing with execution to /post/:id route 
      next();
   }else{
      next(new Error('Incorrect id'));
   }
});

// ...

app.get("/post/:id", function(request, response){
   ...
});