Custom validation rules in Express

Introduction

Custom validation rules help you check user input in ways built-in checks can't. This keeps your app safe and working right.

You want to check if a username is unique before saving.

You need to verify a password meets special rules like including symbols.

You want to confirm a date is not in the past.

You want to validate a custom ID format that built-in validators don't support.

Syntax

Express

const { body, validationResult } = require('express-validator');

app.post('/route', [
  body('fieldName').custom(value => {
    if (!yourCheck(value)) {
      throw new Error('Custom error message');
    }
    return true;
  })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.send('Success');
});

The custom method takes a function that returns true if valid or throws an error if invalid.

Always return true when validation passes to signal success.

Examples

This checks if the age is 18 or older.

Express

body('age').custom(value => {
  if (value < 18) {
    throw new Error('Must be at least 18');
  }
  return true;
})

This example shows an async check to see if a username is already used.

Express

body('username').custom(async value => {
  const userExists = await checkUserInDatabase(value);
  if (userExists) {
    throw new Error('Username already taken');
  }
  return true;
})

Sample Program

This Express app has a route '/register' that checks if the password includes at least one number. If not, it sends an error. Otherwise, it confirms success.

Express

const express = require('express');
const { body, validationResult } = require('express-validator');

const app = express();
app.use(express.json());

// Custom validation to check if password has a number
app.post('/register', [
  body('password').custom(value => {
    if (!/\d/.test(value)) {
      throw new Error('Password must contain a number');
    }
    return true;
  })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.send('Registration successful');
});

app.listen(3000);

Important Notes

Custom validators can be synchronous or asynchronous (return a promise).

Always handle validation errors by checking validationResult(req).

Use clear error messages to help users fix input mistakes.

Summary

Custom validation rules let you check inputs beyond built-in checks.

Use custom() with a function that throws errors on invalid input.

Always return true when validation passes.

Practice

1. What is the main purpose of using custom() in Express validation?

easy

A. To format the response JSON

B. To automatically sanitize all inputs

C. To connect to the database

D. To create your own rules for checking input values

Solution

1. Step 1: Understand the role of custom()

   The custom() method allows you to write your own validation logic beyond built-in checks.

2. Step 2: Identify the purpose in input validation

   It is used to check inputs with rules you define, like checking a password strength or a special format.

3. Final Answer:

   To create your own rules for checking input values -> Option D

4. Quick Check:

   Custom validation = custom rules [OK]

Hint: Custom means you write your own check function [OK]

Common Mistakes:

• Thinking custom() sanitizes inputs automatically
• Confusing custom() with response formatting
• Assuming custom() connects to databases

2. Which of the following is the correct syntax to add a custom validation rule using Express Validator?

easy

A. check('age').custom(value => { if(value < 18) throw new Error('Too young'); return true; })

B. check('age').custom(value => value < 18 ? true : false)

C. check('age').custom(value => { return false; })

D. check('age').custom(value => { throw 'Error'; })

Solution

1. Step 1: Review correct custom validation syntax

   The function inside custom() should throw an error if validation fails and return true if it passes.

2. Step 2: Analyze each option

   check('age').custom(value => { if(value < 18) throw new Error('Too young'); return true; }) throws an error when value is less than 18 and returns true otherwise, which is correct. check('age').custom(value => value < 18 ? true : false) returns true when value is less than 18, which is opposite logic. check('age').custom(value => { return false; }) always returns false, which fails validation. check('age').custom(value => { throw 'Error'; }) throws an error unconditionally, so it always fails.

3. Final Answer:

   check('age').custom(value => { if(value < 18) throw new Error('Too young'); return true; }) -> Option A

4. Quick Check:

   Throw error on fail, return true on pass [OK]

Hint: Throw error to fail, return true to pass [OK]

Common Mistakes:

• Returning false instead of throwing error
• Throwing error without condition
• Returning true on invalid input

3. Given this code snippet, what will be the validation result if req.body.username is "abc"?

check('username').custom(value => {
  if(value.length < 5) throw new Error('Too short');
  return true;
})

medium

A. Validation fails with 'Too short' error

B. Validation passes

C. Validation fails with syntax error

D. Validation passes but logs a warning

Solution

1. Step 1: Check the input value length

   The input "abc" has length 3, which is less than 5.

2. Step 2: Apply the custom validation logic

   The function throws an error 'Too short' if length is less than 5, so it throws an error here causing validation to fail.

3. Final Answer:

   Validation fails with 'Too short' error -> Option A

4. Quick Check:

   Input too short = error thrown [OK]

Hint: Check input length against condition in custom() [OK]

Common Mistakes:

• Assuming validation passes for short input
• Confusing error throwing with warnings
• Expecting syntax errors from valid code

4. Identify the error in this custom validation code:

check('email').custom(value => {
  if(!value.includes('@'))
    return new Error('Invalid email');
  return true;
})

medium

A. The function must return false instead of true

B. The condition should check for '.' instead of '@'

C. It should throw an error, not return it

D. No error, code is correct

Solution

1. Step 1: Understand error signaling in custom validation

   Custom validators must throw an error to indicate failure, not return an Error object.

2. Step 2: Analyze the given code

   The code returns new Error('Invalid email') instead of throwing it, so validation will not fail as expected.

3. Final Answer:

   It should throw an error, not return it -> Option C

4. Quick Check:

   Throw error to fail validation [OK]

Hint: Throw errors, don't return them in custom() [OK]

Common Mistakes:

• Returning Error object instead of throwing
• Checking wrong condition for email
• Returning false instead of throwing error

5. You want to create a custom validation rule that checks if a password contains at least one uppercase letter, one number, and is at least 8 characters long. Which of these implementations correctly achieves this?

hard

A. check('password').custom(value => {
if(value.length < 8) return false;
if(!/[A-Z]/.test(value)) return false;
if(!/\d/.test(value)) return false;
return true;
})

B. check('password').custom(value => {
if(!/[A-Z]/.test(value)) throw new Error('Missing uppercase');
if(!/\d/.test(value)) throw new Error('Missing number');
if(value.length < 8) throw new Error('Too short');
return true;
})

C. check('password').custom(value => {
if(value.length < 8) throw 'Too short';
if(!/[A-Z]/.test(value)) throw 'Missing uppercase';
if(!/\d/.test(value)) throw 'Missing number';
return false;
})

D. check('password').custom(value => {
if(value.length >= 8 && /[A-Z]/.test(value) && /\d/.test(value)) return true;
else return false;
})

Solution

1. Step 1: Check each condition with proper error throwing

   check('password').custom(value => { if(!/[A-Z]/.test(value)) throw new Error('Missing uppercase'); if(!/\d/.test(value)) throw new Error('Missing number'); if(value.length < 8) throw new Error('Too short'); return true; }) checks each condition separately and throws a specific error if it fails, returning true only if all pass.

2. Step 2: Compare other options for correctness

   check('password').custom(value => { if(value.length < 8) return false; if(!/[A-Z]/.test(value)) return false; if(!/\d/.test(value)) return false; return true; }) returns false instead of throwing errors, which is incorrect. check('password').custom(value => { if(value.length < 8) throw 'Too short'; if(!/[A-Z]/.test(value)) throw 'Missing uppercase'; if(!/\d/.test(value)) throw 'Missing number'; return false; }) throws string errors and returns false at the end, which breaks the rule of returning true on success. check('password').custom(value => { if(value.length >= 8 && /[A-Z]/.test(value) && /\d/.test(value)) return true; else return false; }) returns false instead of throwing an error if conditions fail, which is incorrect.

3. Final Answer:

   check('password').custom(value => { if(!/[A-Z]/.test(value)) throw new Error('Missing uppercase'); if(!/\d/.test(value)) throw new Error('Missing number'); if(value.length < 8) throw new Error('Too short'); return true; }) -> Option B

4. Quick Check:

   Throw specific errors, return true if all pass [OK]

Hint: Throw specific errors for each fail, return true if all pass [OK]

Common Mistakes:

• Returning false instead of throwing errors
• Throwing strings instead of Error objects
• Returning false on success