What is the after throw?

Remaining statements skipped; nearest catch up the call stack runs.

Always runs before control leaves the statement, caught or not.

What is the return in finally?

Overrides a pending return or throw from try/catch.

What is the what can be thrown?

Any expression; catch gets exactly that value.

What is the error properties?

name, message; stack is de facto in all major engines.

What is the window error event?

Script errors thrown synchronously (not promise rejections).

Debugging and Error Handling7% of the exam

Throwing, catching and handling errors

How throw, try, catch and finally move control (including return in finally and rethrowing from nested handlers), why to throw Error objects, how to tell TypeError, RangeError, SyntaxError and ReferenceError apart with instanceof, where to place a handler for loops and async code, and when the global error and unhandledrejection events fire.

Official exam objective

Given a scenario, handle errors properly.

~6 min read · 11 practice questions · updated Oct 16, 2026

What the exam expects

Trace throw, try, catch and finally, including nested handlers, rethrowing, and return inside finally.
Throw Error objects (or subclasses) rather than strings or plain objects, and read name, message and stack.
Identify which built-in error a failure raises (TypeError, RangeError, SyntaxError, ReferenceError) and branch on it with instanceof in the right order.
Place the handler where it can actually catch: per item inside a loop, around an await, or with .catch(); use error and unhandledrejection as last-resort global hooks.
Key terms: exception (a thrown value that unwinds the call stack); catch binding (the err in catch (err), holding exactly what was thrown); finally (cleanup that runs however the try exits); rethrow (throwing again from a catch so an outer handler sees it); rejection (the async form of a thrown error).

Key facts

Fact	Detail	Source
After `throw`	Remaining statements skipped; nearest `catch` up the call stack runs	throw (opens in a new tab)
`finally`	Always runs before control leaves the statement, caught or not	try...catch (opens in a new tab)
`return` in `finally`	Overrides a pending `return` or `throw` from `try`/`catch`	try...catch (opens in a new tab)
What can be thrown	Any expression; `catch` gets exactly that value	Control flow and error handling (opens in a new tab)
`Error` properties	`name`, `message`; `stack` is de facto in all major engines	Error.prototype.stack (opens in a new tab)
Invalid JSON string	`JSON.parse` throws `SyntaxError`	JSON.parse() (opens in a new tab)
Undeclared variable	`ReferenceError`	ReferenceError (opens in a new tab)
Window `error` event	Script errors thrown synchronously (not promise rejections)	error event (opens in a new tab)
`unhandledrejection`	Promise rejected with no handler (not fired for rejections from a cross-origin script); `event.reason` holds the value	unhandledrejection event (opens in a new tab)

After throw

Detail: Remaining statements skipped; nearest catch up the call stack runs
Source: throw (opens in a new tab)

finally

Detail: Always runs before control leaves the statement, caught or not
Source: try...catch (opens in a new tab)

return in finally

Detail: Overrides a pending return or throw from try/catch
Source: try...catch (opens in a new tab)

What can be thrown

Detail: Any expression; catch gets exactly that value
Source: Control flow and error handling (opens in a new tab)

Error properties

Detail: name, message; stack is de facto in all major engines
Source: Error.prototype.stack (opens in a new tab)

Invalid JSON string

Detail: JSON.parse throws SyntaxError
Source: JSON.parse() (opens in a new tab)

Undeclared variable

Detail: ReferenceError
Source: ReferenceError (opens in a new tab)

Window error event

Detail: Script errors thrown synchronously (not promise rejections)
Source: error event (opens in a new tab)

unhandledrejection

Detail: Promise rejected with no handler (not fired for rejections from a cross-origin script); event.reason holds the value
Source: unhandledrejection event (opens in a new tab)

Sources for this section (8)

Check yourself

Why should code throw `new Error('…')` rather than a string?
When does a `return` in `finally` change a function's result?
Why does `err instanceof Error` match a `TypeError`?
Which window event reports an error thrown inside an async function that nobody awaited?

The questions are free to read. Answers with their sources and self-grading are included with the pack.

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What the exam expects

Key facts

Check yourself

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