How to extend Error with custom properties?

JavaScriptJunior~1 min read

1. The main idea

You create your own class that inherits from Error, and add the properties you need to it - for example: code, status, details, cause, context, and so on.

2. Example: a basic custom error

javascript

class AppError extends Error {
  constructor(message, code) {
    super(message);            // pass the message to the parent (Error)
    this.name = "AppError";    // set the error's name
    this.code = code;          // add our own property
  }
}

try {
  throw new AppError("Something went wrong", "SERVER_ERROR");
} catch (e) {
  console.log(e.name);     // AppError
  console.log(e.message);  // Something went wrong
  console.log(e.code);     // SERVER_ERROR
  console.log(e.stack);    // Call stack
}

Now your error has both the standard properties (message, stack) and custom ones (code).

3. Example: adding an HTTP code and context

This is convenient for APIs, especially in NestJS / Express:

javascript

class HttpError extends Error {
  constructor(message, status = 500, details = {}) {
    super(message);
    this.name = this.constructor.name;
    this.status = status;
    this.details = details;
    if (Error.captureStackTrace) {
      Error.captureStackTrace(this, this.constructor);
    }
  }
}

throw new HttpError("User not found", 404, { userId: 123 });

When handling the error:

javascript

try {
  throw new HttpError("User not found", 404, { userId: 123 });
} catch (err) {
  console.log(err.name);     // HttpError
  console.log(err.status);   // 404
  console.log(err.details);  // { userId: 123 }
}

4. Example: a hierarchy of custom errors

You can build a whole "tree" of errors:

javascript

class AppError extends Error {
  constructor(message, code) {
    super(message);
    this.name = this.constructor.name;
    this.code = code;
    Error.captureStackTrace?.(this, this.constructor);
  }
}

class ValidationError extends AppError {
  constructor(message, field) {
    super(message, "VALIDATION_ERROR");
    this.field = field;
  }
}

class AuthError extends AppError {
  constructor(message = "Unauthorized") {
    super(message, "AUTH_ERROR");
    this.status = 401;
  }
}

Usage:

javascript

try {
  throw new ValidationError("Invalid email", "email");
} catch (err) {
  if (err instanceof ValidationError) {
    console.log("Error in field:", err.field);
  }
  console.log(err.code); // VALIDATION_ERROR
}

5. Example with `cause` (ES2022+)

The modern ECMAScript standard supports the cause property, to store the original error that led to the current one.

javascript

try {
  JSON.parse("invalid JSON");
} catch (parseErr) {
  throw new AppError("Error parsing JSON", "PARSE_ERROR", { cause: parseErr });
}

To support cause, you need to add it to the constructor:

javascript

class AppError extends Error {
  constructor(message, code, options = {}) {
    super(message, options);
    this.name = this.constructor.name;
    this.code = code;
    this.cause = options.cause;
  }
}

6. Example of usage in production (Express / NestJS)

javascript

app.use((err, req, res, next) => {
  if (err instanceof AppError) {
    res.status(err.status || 500).json({
      error: err.name,
      message: err.message,
      code: err.code,
      details: err.details,
    });
  } else {
    res.status(500).json({ error: "InternalError", message: "Unknown error" });
  }
});

7. Why extend Error

Reason	Example
Clear, understandable messages	`"Email is required"` instead of `"Unexpected token"`
Structure and typing	`ValidationError`, `AuthError`, `HttpError`
Convenient logging	You can include `code`, `status`, `details`
Storing the original cause	via `cause`
Easier debugging	Different error types are easy to tell apart in `catch`

Summary

To extend Error with your own properties:

javascript

class MyError extends Error {
  constructor(message, customProp) {
    super(message);
    this.name = "MyError";
    this.customProp = customProp;
    Error.captureStackTrace?.(this, this.constructor);
  }
}

And use it:

javascript

throw new MyError("Database error", { query: "SELECT * FROM users" });

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