Validate an id with Express middleware, part 2

Run middleware methods as callbacks

In the previous article, we saw how to create a validateId middleware method that checks if a record exists in a database.

If the record is not found, validateId sends a response back to the client with a 404 error, and no further processing occurs.

If the record is found, though, the method saves the record, as a Javascript object, inside the request object. It then sends the request further down the line to be processed by other methods in the chain that require the found record.

We have defined validateId and called it at the beginning of the file, with server.use(validateId), so it's available to successive methods.

One possible problem

One problem we have with this approach is that validateId will be run for all methods defined after it, but successive methods may not actually need any found record for their purposes, so they don't need to validate an id.

For example, we have one method, later in our chain, that displays all the toys and doesn't require any specific toy id. Since this method doesn't need an id, the field is not present in the URL:

server.get('/toys', (req, res) => {...})  // no :id

Since validateId is higher in the methods chain, the '/toys' request gets to it before it has a chance to trigger server.get('/toys').

The id field is missing from '/toys', so validateId stops processing the request and returns an error. server.get('/toys') never even sees this request.

Solving the problem

To solve this problem, we need a way to pick and choose which middleware method gets a chance to run validateId and which one skips it.

As it turns out, like many other things with Express, the solution to this problem is pretty easy. We can add an additional argument to the server.get() method, after the URL argument, that takes a callback middleware function. This function is processed before the code inside the method is called.

So, we simply add validateId as the second parameter to server.get(). validateId will run and make a database call, save the returned record in the request and provide it to server.get() for further processing.

server.get('/:id', validateId, (req, res) => {
  res.status(200).json(req.toy);
});

Since validateId, already performs a check to see if a record is present, server.get() won't need to do any database call or error handling. Its only responsibility is to return the object found, which is already saved into the request object.

This way of handling middleware methods lets us add the validateId function only to the methods that need it, so we don't need to call server.use(validateId) at the top anymore.

This means that other methods down the line won't be affected by the result of running validateId unnecessarily.

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