Custom Error Classes
In this page:
Creating a Custom Error
A custom error class extends the built-in Error and usually sets this.name to a specific string, which lets logging and error-reporting tools identify exactly which category of failure occurred at a glance.
Example: Creating a Custom Error
class NotFoundError extends Error {
constructor(message: string) {
super(message);
this.name = "NotFoundError";
}
}
try {
throw new NotFoundError("User not found");
} catch (err) {
if (err instanceof NotFoundError) console.log(err.name, err.message);
}Custom Error Properties
Custom errors can carry additional typed properties beyond the standard message — an error code, a resource id, a validation field name — giving catch blocks structured data to act on instead of just parsing a string.
Example: Custom Error Properties
class ValidationError extends Error {
constructor(message: string, public field: string) {
super(message);
this.name = "ValidationError";
}
}
try {
throw new ValidationError("Required", "email");
} catch (err) {
if (err instanceof ValidationError) console.log(err.field, err.message);
}Multiple Custom Errors
Different error classes can represent different failure categories — NotFoundError, ValidationError, AuthError — and be caught and handled separately, so each kind of failure gets an appropriately different response.
Example: Multiple Custom Errors
class NotFoundError extends Error {}
class ValidationError extends Error {}
class AuthError extends Error {}
function handle(err: Error) {
if (err instanceof NotFoundError) console.log("404");
else if (err instanceof ValidationError) console.log("400");
else if (err instanceof AuthError) console.log("401");
}
handle(new ValidationError("bad input"));Custom Errors in Functions
Functions can throw custom errors while callers use instanceof checks to decide how to respond, turning a generic try/catch into a dispatch table over specific, meaningful failure types.
Example: Custom Errors in Functions
class NotFoundError extends Error {}
function findUser(id: number) {
if (id !== 1) throw new NotFoundError("User not found");
return { id, name: "Ravi" };
}
try {
findUser(2);
} catch (err) {
if (err instanceof NotFoundError) console.log("Handled:", err.message);
}Best Practices for Custom Errors
Reserve custom errors for meaningful application-level failures worth distinguishing, and keep their extra properties focused and strongly typed rather than turning every error into a loosely-typed grab bag.
Example: Best Practices for Custom Errors
class PaymentError extends Error {
constructor(message: string, public code: number) {
super(message);
this.name = "PaymentError";
}
}
try {
throw new PaymentError("Card declined", 402);
} catch (err) {
if (err instanceof PaymentError) console.log(err.code, err.message);
}Chapter Quiz — Complete all 5 topics to unlock
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