Custom Error Classes

Example

class AppError extends Error {
  constructor(public readonly code: string, message: string, public readonly status = 500) {
    super(message);
    this.name = new.target.name;
  }
}

class NotFoundError extends AppError {
  constructor(entity: string, id: string | number) {
    super("NOT_FOUND", `${entity} ${id} not found`, 404);
  }
}

class ValidationError extends AppError {
  constructor(public readonly fields: Record<string, string>) {
    super("VALIDATION", "Invalid input", 400);
  }
}

const errors: Error[] = [new NotFoundError("Product", 42), new ValidationError({ price: "must be > 0" }), new Error("boom")];

for (const e of errors) {
  if (e instanceof ValidationError) console.log(e.status, e.name, e.fields);
  else if (e instanceof AppError) console.log(e.status, e.name, e.message);
  else console.log(500, e.name, e.message);
}

Output

404 NotFoundError Product 42 not found
400 ValidationError { price: 'must be > 0' }
500 Error boom

Example

ES2022 added cause to wrap a low-level error inside a meaningful one without losing the original.

function readConfig(text: string): { port: number } {
  try {
    return JSON.parse(text);
  } catch (e) {
    throw new Error("Config file is invalid", { cause: e });
  }
}

try {
  readConfig("{port: 3000");
} catch (e) {
  if (e instanceof Error) {
    console.log(e.message);
    console.log("cause:", e.cause instanceof SyntaxError ? "SyntaxError" : e.cause);
  }
}

Output

Config file is invalid
cause: SyntaxError