Custom error classes in Node.js - Step-by-Step Execution

Concept Flow - Custom error classes

Define class extending Error

↓

Create instance of custom error

↓

Throw custom error

↓

Catch error in try-catch

↓

Check error type with instanceof

↓

Handle or log error

↓

End

This flow shows how to define a custom error class, throw it, catch it, and handle it by checking its type.

Execution Sample

Node.js

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

try {
  throw new MyError('Oops!');
} catch (e) {
  console.log(e.name + ': ' + e.message);
}

Defines a custom error class, throws it, catches it, and logs its name and message.

Execution Table

Step	Action	Evaluation	Result
1	Define class MyError extending Error	Class MyError created	MyError class ready
2	Create new MyError with message 'Oops!'	new MyError('Oops!')	Error object with name 'MyError' and message 'Oops!'
3	Throw the MyError instance	throw new MyError('Oops!')	Exception thrown, jumps to catch
4	Catch error in catch block	catch(e)	e is MyError instance
5	Log e.name + ': ' + e.message	console.log output	MyError: Oops!

💡 Execution stops after catch block finishes handling the thrown error.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
MyError	undefined	Class defined	Class defined	Class defined	Class defined
e	undefined	undefined	undefined	MyError instance	MyError instance

Key Moments - 3 Insights

Why do we set this.name = 'MyError' inside the constructor?

Because the default Error name is 'Error', setting this.name helps identify the error type clearly in logs, as shown in step 5 of the execution_table.

What happens if we throw the error outside a try-catch block?

The program will crash or stop running because the error is unhandled. The try-catch in step 3 and 4 shows how to catch and handle it safely.

How do we check if the caught error is our custom error?

We use 'instanceof MyError' to check the error type, which is possible because the error object is created from the MyError class as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'e.name' at step 5?

A 'MyError'

B 'Error'

C 'Oops!'

D undefined

💡 Hint

Check the 'Result' column at step 5 where the error name is logged.

At which step is the custom error instance created?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column where 'new MyError' is called.

If we remove 'this.name = "MyError"' from the constructor, what changes in the output at step 5?

A The output will not change

B The output will show 'Error: Oops!' instead of 'MyError: Oops!'

C The program will crash

D The error message will be empty

💡 Hint

Refer to the key_moments about setting the error name and the output logged at step 5.

Concept Snapshot

Custom error classes extend Error to create specific error types.
Set this.name in constructor to identify error type.
Throw instances with throw new MyError(message).
Catch with try-catch and check type with instanceof.
Helps organize error handling clearly.

Full Transcript

This lesson shows how to create custom error classes in Node.js by extending the built-in Error class. We define a class MyError that sets its name property to identify the error type. Then we create an instance of MyError with a message and throw it inside a try block. The catch block catches the error object, which is an instance of MyError. We log the error's name and message to show how custom errors can be identified and handled. This helps organize error handling in programs by creating specific error types instead of using generic errors.