12 — Errors

One line: JavaScript lets you throw any value and catch it anywhere up the stack, but only an Error carries a stack and a cause, and an error nobody catches — thrown or rejected — ends a Node process with exit status 1.

The chapter starts with throw and try...catch, and why the thing you throw should be an Error. It then lists the built-in error types and the mistakes that produce each, covers finally and the return inside it that silently wins, and shows how to define your own error classes and wrap one error in another with cause. It ends with what an error records in error.stack, and with what Node does with an error that nobody catches.

The pages

Page	Level	In one line	Status
throw and try...catch	101	throw accepts any value, but only an Error object records a stack: throw a string and catch receives just the string — no stack, no instanceof Error, nothing to say which calls led there.	stub
Built-in error types	101	The engine picks the type: TypeError for using a value the wrong way, ReferenceError for an unknown name, RangeError for a number out of range, SyntaxError for text it cannot parse — and EvalError only when the host forbids eval altogether.	stub
finally	201	finally runs however the try block is left — by return, throw, break or falling off the end — and a return inside it replaces the result and silently discards a thrown error; only ending the process, as process.exit() does, skips it.	stub
Custom errors and cause	201	new Error(message, { cause }) keeps the original error on .cause, so wrapping loses nothing; but a bare class ConfigError extends Error {} still calls itself "Error" — its name comes from Error.prototype until you set your own.	stub
Stack traces	201	V8 fills in error.stack when the Error is created, not when it is thrown, and keeps only the top 10 frames (Error.stackTraceLimit); stack is not in the standard, and an await chain still shows up as at async frames.	stub
Unhandled errors	201	An uncaught exception and an unhandled rejection both end Node with exit status 1, and a rejection is unhandled if no handler is attached by the time the microtask queue empties — a .catch added one timer later is already too late.	stub

Boundaries

Rejected promises and try...catch around await are built up in Async and the event loop; the error event of an EventEmitter is in The Node.js runtime; using and Symbol.dispose are in Metaprogramming.