404 Not Found handler in Express - Step-by-Step Execution

Concept Flow - 404 Not Found handler

Request Received

↓

Match Routes?

Yes→Handle Route

No↓

404 Handler Middleware

↓

Send 404 Response

↓

End Request

When a request comes in, Express checks routes. If none match, it runs the 404 handler middleware to send a 'Not Found' response.

Execution Sample

Express

app.use((req, res) => {
  res.status(404).send('Page not found');
});

This code adds a middleware that catches all unmatched requests and sends a 404 status with a message.

Execution Table

Step	Request URL	Route Match?	Action	Response Status	Response Body
1	/home	Yes	Handle /home route	200	Home page content
2	/about	Yes	Handle /about route	200	About page content
3	/unknown	No	Run 404 handler middleware	404	Page not found
4	-	-	Request ends	-	-

💡 Request ends after sending 404 response for unmatched URL

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Request URL	/home	/about	/unknown	/unknown	/unknown
Response Status	200	200	404	404	404
Response Body		Home page content	About page content	Page not found	Page not found

Key Moments - 2 Insights

Why does the 404 handler run only when no routes match?

Express processes routes in order. The 404 handler is last and runs only if no previous route matched, as shown in execution_table step 3.

What happens if the 404 handler is placed before routes?

It would catch all requests first, preventing routes from running. The execution_table shows correct order with 404 last.

Visual Quiz - 3 Questions

Look at the execution table, what is the response status when the URL is '/unknown'?

A 500

B 200

C 404

D 302

💡 Hint

Check the row where Request URL is '/unknown' in the execution_table.

At which step does Express decide no route matches and runs the 404 handler?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for 'Run 404 handler middleware' in the Action column of execution_table.

If you add a new route for '/unknown' before the 404 handler, what changes in the execution table?

A Step 4 would send 404 response

B Step 3 would show 'Handle /unknown route' with status 200

C Step 3 would still run 404 handler

D No change in execution table

💡 Hint

Think about how route matching affects which middleware runs, referencing variable_tracker and execution_table.

Concept Snapshot

Express 404 Handler:
- Placed after all routes
- Catches unmatched requests
- Sends res.status(404).send('Page not found')
- Ensures user sees clear 'Not Found' message
- Order matters: must be last middleware

Full Transcript

In Express, when a request comes in, the server checks each route in order. If a route matches the request URL, it handles the request and sends a response. If no route matches, Express runs the 404 Not Found handler middleware. This middleware sets the response status to 404 and sends a message like 'Page not found'. This process ensures users get a clear message when they visit a URL that does not exist on the server. The 404 handler must be placed after all other routes to work correctly. If placed earlier, it would block other routes from running. The execution table shows requests to known routes get a 200 status and content, while unknown routes get a 404 status and the not found message.