Custom Error classes

When to make your own

Built-in classes (TypeError, RangeError) cover language-level errors. For domain errors — "user not found", "rate limited", "stock empty" — define your own:

class NotFoundError extends Error {
  constructor(what) {
    super(\`${what} not found\`);
    this.name = "NotFoundError";
  }
}

class RateLimitError extends Error {
  constructor(retryAfter) {
    super(\`rate limited, retry in ${retryAfter}s\`);
    this.name = "RateLimitError";
    this.retryAfter = retryAfter;
  }
}

Why this scales

• Call sites can if (err instanceof RateLimitError) await sleep(err.retryAfter * 1000).
• Logging libraries group by err.name.
• Adding new fields (.userId, .requestId) attaches structured context to the error.

Set this.name in the constructor

If you forget this.name = "...", the error logs as plain "Error: msg" — useless for grouping. Always set it.