Validation Errors — Field-Level Error Reporting

In this tutorial, you will learn about Validation Errors. We cover key concepts, practical examples, and best practices to help you master this topic.

Validation errors report which specific fields failed validation and why, using structured error details that clients can parse programmatically to fix individual fields.

Validation Error Format

{
  "status": 422,
  "error": "validation_error",
  "message": "Validation failed for the request",
  "details": [
    {
      "field": "email",
      "code": "invalid_format",
      "message": "Must be a valid email address",
      "value": "not-an-email"
    },
    {
      "field": "password",
      "code": "too_short",
      "message": "Minimum 8 characters required"
    }
  ]
}

Implementation

function validateCreateUser(body) {
  const errors = [];

  if (!body.name || body.name.trim().length < 2) {
    errors.push({
      field: "name",
      code: "too_short",
      message: "Name must be at least 2 characters",
      value: body.name
    });
  }

  if (!body.email || !isValidEmail(body.email)) {
    errors.push({
      field: "email",
      code: "invalid_format",
      message: "A valid email address is required",
      value: body.email
    });
  }

  return errors.length > 0 ? errors : null;
}

Common Mistakes

1. Single error at a time — Return all validation errors at once, not one by one.
2. Missing field name — Tell the client which field failed.
3. Missing received value — Help clients debug what they sent wrong.
4. Generic messages — Give specific reasons (too_short, invalid_format, already_taken).

Practice Questions

1. What fields should each validation error include?
2. Why return all errors at once instead of one at a time?
3. What is the purpose of the code field?

Challenge

Implement comprehensive validation for a user registration endpoint. Validate name, email, password, age, and terms acceptance. Return all errors at once.

What's Next

In the next lesson, you will learn authentication error handling.