Controller pattern for route handlers in Express - Step-by-Step Execution

Concept Flow - Controller pattern for route handlers

Client sends HTTP request

↓

Express Router receives request

↓

Router calls Controller function

↓

Controller processes request

↓

Controller sends response

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Client receives response

The client sends a request, Express router directs it to a controller function, which processes and sends back a response.

Execution Sample

Express

import express from 'express';
const app = express();

// Controller function
const getUser = (req, res) => {
  res.send(`User ID: ${req.params.id}`);
};

app.get('/user/:id', getUser);

This code sets up a route '/user/:id' that uses a controller function to send back the user ID from the URL.

Execution Table

Step	Action	Input	Controller State	Response Sent
1	Client sends GET /user/42	GET /user/42	Waiting	No
2	Express router matches route	/user/42	Waiting	No
3	Router calls getUser controller	/user/42	Processing req.params.id = '42'	No
4	Controller executes res.send	/user/42	Processed id '42'	Yes: 'User ID: 42'
5	Response sent to client	User ID: 42	Done	Yes

💡 Response sent, request cycle complete

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
req.params.id	undefined	'42'	'42'	'42'
responseSent	false	false	true	true

Key Moments - 3 Insights

Why does the controller get called after the router matches the route?

Because the router directs matching requests to the controller function as shown in execution_table step 3.

What does res.send do inside the controller?

It sends the response back to the client and ends the request cycle, as seen in execution_table step 4.

Why is req.params.id available inside the controller?

Express parses the URL parameters before calling the controller, so req.params.id holds the value from the URL, shown in variable_tracker after step 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of req.params.id at step 3?

A undefined

B 'user'

C '42'

D null

💡 Hint

Check the 'Controller State' column at step 3 in the execution_table.

At which step does the response get sent to the client?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Response Sent' column in the execution_table.

If the route was '/user/:id/profile', how would the controller receive the id parameter?

A In req.params.profile

B In req.params.id as usual

C In req.query.id

D It would not be available

💡 Hint

Route parameters are always available in req.params by their name, as shown in variable_tracker.

Concept Snapshot

Controller pattern in Express:
- Define controller functions to handle requests
- Use Express router to map routes to controllers
- Controllers receive req and res objects
- Controllers process data and send responses
- Keeps route logic clean and organized

Full Transcript

In Express, the controller pattern means separating the code that handles requests into functions called controllers. When a client sends a request, Express router matches the route and calls the appropriate controller function. The controller receives the request and response objects, processes the request (like reading URL parameters), and sends back a response. This keeps the code organized and easy to manage. For example, a route '/user/:id' uses a controller that reads req.params.id and sends it back in the response. The execution table shows each step from receiving the request to sending the response. Variables like req.params.id change from undefined to the actual value as the request is processed.