Error Handling & Custom ErrorsAdvanced
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javascriptHow do you create Custom Error Classes in JavaScript and why are they beneficial?
Explanation & Concept
Custom error classes allow applications to classify and differentiate errors based on business logic (e.g. `ValidationError`, `AuthenticationError`, `NotFoundError`).
You create them by extending the native `Error` class, calling `super(message)`, and setting the `name` property. Handlers can then use `instanceof` to route errors appropriately.
Code Example
javascript
class ValidationError extends Error {
constructor(message, field) {
super(message);
this.name = "ValidationError";
this.field = field;
}
}
function registerUser(username) {
if (!username) {
throw new ValidationError("Username is required", "username");
}
return { username, registered: true };
}
try {
registerUser("");
} catch (err) {
if (err instanceof ValidationError) {
console.error(`Validation failed on field "${err.field}": ${err.message}`);
} else {
console.error("Unexpected error:", err);
}
}How It Works & Key Takeaways
`instanceof` checks allow fine-grained error recovery: user validation errors can render a form alert, while unexpected errors trigger logging and alerts.
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