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.