Custom validation rules in Express - Step-by-Step Execution

Concept Flow - Custom validation rules

Receive request data

↓

Apply built-in validations

↓

Apply custom validation function

↓

Is data valid?

No→Send error response

Yes↓

Proceed to next middleware or handler

The flow shows how Express receives data, applies built-in and custom validations, then either sends errors or continues processing.

Execution Sample

Express

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

app.post('/user', [
  body('age').custom(value => {
    if (value < 18) throw new Error('Must be 18 or older');
    return true;
  })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) return res.status(400).json({ errors: errors.array() });
  res.send('User valid');
});

This code adds a custom validation to check if age is 18 or older, then sends errors or success.

Execution Table

Step	Action	Input Value	Validation Result	Next Step
1	Receive request with age=16	16	Not validated yet	Apply built-in validations
2	Apply built-in validations	16	Pass (no built-in rules here)	Apply custom validation
3	Run custom validation function	16	Fail: 'Must be 18 or older'	Send error response
4	Send error response	N/A	Response 400 with error message	End
5	Receive request with age=20	20	Not validated yet	Apply built-in validations
6	Apply built-in validations	20	Pass	Apply custom validation
7	Run custom validation function	20	Pass	Proceed to handler
8	Send success response	N/A	Response 200 'User valid'	End

💡 Execution stops after sending response: either error if validation fails or success if passes.

Variable Tracker

Variable	Start	After Step 3 (age=16)	After Step 7 (age=20)	Final
age	undefined	16	20	20 or 16 depending on request
validationResult.errors	empty	[{msg:'Must be 18 or older'}]	empty	Errors or empty array
response.status	unset	400	200	Final HTTP status code
response.body	unset	{"errors":[{"msg":"Must be 18 or older"}]}	"User valid"	Final response content

Key Moments - 3 Insights

Why does the custom validation function throw an error instead of returning false?

Throwing an Error inside the custom validator signals express-validator that validation failed with a message. Returning false would not provide the error message. See execution_table row 3.

What happens if validationResult finds errors?

If errors exist, the code sends a 400 response with error details and stops further processing. See execution_table row 4.

Can custom validation access other request fields?

Yes, the custom validator can access req object if needed, but here it only checks the value directly. This is common for cross-field checks.

Visual Quiz - 3 Questions

Look at the execution table, what validation result occurs at step 3 when age is 16?

A Fail with message 'Must be 18 or older'

B Pass

C No validation performed

D Throws a system error

💡 Hint

Check the 'Validation Result' column in row 3 of the execution_table.

At which step does the server send a success response for age 20?

A Step 4

B Step 8

C Step 6

D Step 7

💡 Hint

Look for 'Send success response' action in the execution_table.

If the custom validator returned false instead of throwing an error, what would happen?

A Validation would fail with the same error message

B Validation would pass silently

C Validation would fail but no error message would be shown

D Server would crash

💡 Hint

Recall key_moments explanation about throwing Error vs returning false.

Concept Snapshot

Custom validation rules in Express use functions that throw errors to signal validation failure.
Use express-validator's body().custom() to add these rules.
If validation fails, send error response with details.
If passes, continue to next middleware or handler.
This allows flexible checks beyond built-in validators.

Full Transcript

This visual execution trace shows how Express handles custom validation rules using express-validator. First, the server receives request data. Built-in validations run, then custom validation functions check specific conditions. If the custom validator throws an error, validation fails and the server sends a 400 error response with messages. If validation passes, the server proceeds to handle the request normally and sends a success response. Variables like age, validation errors, and response status update step-by-step. Key moments clarify why throwing errors is needed for validation failure and how errors stop request processing. The quiz tests understanding of validation results and response steps. This helps beginners see exactly how custom validation works in Express.

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