Custom Error Classes in JavaScript

Custom Error subclasses create typed error hierarchies enabling instanceof checks, automatic HTTP status mapping, and consistent serialization for REST/GraphQL APIs. Eliminates string-based error code matching while providing structured metadata. Essential for middleware error handling and client-side error boundaries with precise catch blocks.

Code Example:-

Code

class ApiError extends Error {
  constructor(message, code, status = 500, details = {}) {
    super(message);
    this.name = this.constructor.name;
    this.code = code;
    this.status = status;
    this.details = details;
  }
}

class ValidationError extends ApiError {
  constructor(field, value) {
    super(`Invalid ${field}: ${value}`, 'VALIDATION_ERROR', 400, { field, value });
  }
}

class NotFoundError extends ApiError {
  constructor(resource) {
    super(`${resource} not found`, 'NOT_FOUND', 404);
  }
}

// Usage in API handler
try {
  const user = await getUser(id);
} catch (error) {
  if (error instanceof ValidationError) {
    return Response.json({ error: error.message }, { status: 400 });
  }
  if (error instanceof NotFoundError) {
    return Response.json({ error: error.message }, { status: 404 });
  }
  throw error;
}

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How do custom error class hierarchies improve type-safe API error handling in modern JavaScript applications?

Code Example:-

Code

class ApiError extends Error {
  constructor(message, code, status = 500, details = {}) {
    super(message);
    this.name = this.constructor.name;
    this.code = code;
    this.status = status;
    this.details = details;
  }
}

class ValidationError extends ApiError {
  constructor(field, value) {
    super(`Invalid ${field}: ${value}`, 'VALIDATION_ERROR', 400, { field, value });
  }
}

class NotFoundError extends ApiError {
  constructor(resource) {
    super(`${resource} not found`, 'NOT_FOUND', 404);
  }
}

// Usage in API handler
try {
  const user = await getUser(id);
} catch (error) {
  if (error instanceof ValidationError) {
    return Response.json({ error: error.message }, { status: 400 });
  }
  if (error instanceof NotFoundError) {
    return Response.json({ error: error.message }, { status: 404 });
  }
  throw error;
}

How do custom error class hierarchies improve type-safe API error handling in modern JavaScript applications?

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