Custom error classes in Express - Step-by-Step Execution

Concept Flow - Custom error classes

Define Custom Error Class

↓

Throw Custom Error in Route

↓

Express Error Middleware Catches Error

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Send Custom Error Response to Client

This flow shows how to create a custom error class, throw it in a route, catch it in Express error middleware, and send a response.

Execution Sample

Express

class NotFoundError extends Error {
  constructor(message) {
    super(message);
    this.name = 'NotFoundError';
    this.statusCode = 404;
  }
}

app.get('/item/:id', (req, res, next) => {
  if (!db.has(req.params.id)) {
    return next(new NotFoundError('Item not found'));
  }
  res.send(db.get(req.params.id));
});

Defines a custom error class and uses it in an Express route to handle missing items.

Execution Table

Step	Action	Error Instance	Middleware Called	Response Sent
1	Request to /item/123	None	No	No
2	Check if item exists in db	None	No	No
3	Item not found, create NotFoundError	NotFoundError('Item not found')	No	No
4	Call next() with error	NotFoundError('Item not found')	Yes	No
5	Error middleware receives NotFoundError	NotFoundError('Item not found')	Yes	No
6	Send response with status 404 and message	NotFoundError('Item not found')	Yes	Yes
7	Request ends with 404 response	None	No	Yes

💡 Response sent with 404 status because item was not found.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
error	undefined	NotFoundError('Item not found')	NotFoundError('Item not found')	undefined

Key Moments - 2 Insights

Why do we call next() with the error instead of sending a response directly?

Calling next(error) passes the error to Express error middleware, which centralizes error handling. See execution_table step 4 and 5.

What is the purpose of setting this.name and this.statusCode in the custom error?

this.name helps identify the error type, and this.statusCode lets middleware send the correct HTTP status. See execution_sample code.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Express error middleware first handle the error?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Check the 'Middleware Called' column in the execution_table.

According to variable_tracker, what is the value of 'error' after step 4?

A NotFoundError('Item not found')

B undefined

C null

D Error('Generic error')

💡 Hint

Look at the 'After Step 4' column for 'error' in variable_tracker.

If we did not set this.statusCode in the custom error, what would likely happen?

A The error would not be caught by middleware

B Express would send a 404 status anyway

C Express would send a default 500 status

D The server would crash

💡 Hint

Think about default HTTP status codes when no statusCode is set.

Concept Snapshot

Custom error classes extend Error to add properties like statusCode.
Throw them in routes with next(error) to trigger Express error middleware.
Middleware reads error properties to send proper HTTP responses.
This centralizes error handling and keeps routes clean.

Full Transcript

This visual execution trace shows how to create and use custom error classes in Express. First, a custom error class NotFoundError is defined extending the built-in Error class, adding a statusCode property. When a request comes in for an item, the route checks if the item exists. If not, it creates a NotFoundError instance and calls next() with it. Express then calls the error-handling middleware, which reads the error's statusCode and message to send a 404 response to the client. The variable tracker shows the error variable holding the NotFoundError instance during middleware handling. Key moments clarify why next(error) is used and why setting statusCode matters. The quiz tests understanding of middleware timing, variable state, and default error behavior.