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How do you create a custom error type?

JavaScriptJunior~1 min read
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Basic template (ES2022+)

javascript
class AppError extends Error { constructor(message, { code, cause } = {}) { super(message, { cause }); // preserves stack and cause this.name = this.constructor.name; // "AppError" this.code = code ?? "APP_ERROR"; if (Error.captureStackTrace) { Error.captureStackTrace(this, this.constructor); } } } // Specific cases class ValidationError extends AppError { constructor(message, details, opts = {}) { super(message, { ...opts, code: "VALIDATION_ERROR" }); this.details = details; // any fields you need } } class AuthError extends AppError { constructor(message = "Unauthorized", opts = {}) { super(message, { ...opts, code: "AUTH_ERROR" }); this.status = 401; } }

Usage:

javascript
function parseUser(input) { if (!input.email) { throw new ValidationError("Email is required", { field: "email" }); } return input; } try { parseUser({}); } catch (e) { if (e instanceof ValidationError) { console.log(e.code, e.details); // VALIDATION_ERROR { field: "email" } } else { console.error("Unexpected error", e); } }

Wrapping the original error (cause)

Useful when catching low-level exceptions:

javascript
try { JSON.parse("invalid JSON"); } catch (e) { throw new AppError("Failed to parse the config", { cause: e, code: "CONFIG_PARSE" }); }

Later you can inspect the chain:

javascript
console.error(err.cause); // the original error

TypeScript version

javascript
type AppErrorCode = "APP_ERROR" | "VALIDATION_ERROR" | "AUTH_ERROR"; class AppError extends Error { code: AppErrorCode; declare cause?: unknown; // so TS knows about cause constructor(message: string, opts: { code?: AppErrorCode; cause?: unknown } = {}) { super(message, { cause: opts.cause }); this.name = new.target.name; this.code = opts.code ?? "APP_ERROR"; if ((Error as any).captureStackTrace) { (Error as any).captureStackTrace(this, new.target); } } } class ValidationError extends AppError { details?: Record<string, unknown>; constructor(message: string, details?: Record<string, unknown>, opts: { cause?: unknown } = {}) { super(message, { ...opts, code: "VALIDATION_ERROR" }); this.details = details; } }

A few practical tips

  • Serialization (logging/API response):

    javascript
    function errorToJson(err) { return { name: err.name, message: err.message, code: err.code, stack: err.stack, details: err.details, cause: err.cause instanceof Error ? err.cause.message : err.cause, }; }

  • HTTP mapping: Add a status (400/401/404/500) to your errors and map them to responses at the middleware level.

  • Guarantee instanceof: Modern engines are fine, but the extends Error + captureStackTrace (when available) pattern is a reliable base.

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