Custom validation rules in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom validation rule in Express?

A custom validation rule is a user-defined function that checks if input data meets specific conditions beyond built-in validations.

beginner

How do you add a custom validation rule using express-validator?

Use the .custom() method on a validation chain and provide a function that returns true or throws an error if validation fails.

intermediate

What should a custom validation function return or throw?

It should return true if validation passes or throw an error (or return a rejected promise) if validation fails.

intermediate

Why use custom validation rules instead of built-in ones?

Custom rules let you check complex or unique conditions specific to your app that built-in validators can't handle.

beginner

Give an example of a simple custom validation rule in Express.

Example: .custom(value => { if (!value.startsWith('A')) throw new Error('Must start with A'); return true; }) checks if input starts with 'A'.

Which method is used to create a custom validation rule in express-validator?

A .validate()

B .custom()

C .checkCustom()

D .rule()

The .custom() method allows you to define your own validation logic.

What should a custom validation function do if the input is invalid?

A Ignore the input

B Return true

C Return false silently

D Throw an error or return a rejected promise

Throwing an error or returning a rejected promise signals validation failure.

Why might you need a custom validation rule?

A To handle unique or complex input checks not covered by built-in validators

B To speed up server response

C To check simple required fields

D To replace all built-in validators

Custom rules handle special cases that built-in validators can't.

Which package is commonly used with Express for validation including custom rules?

A express-validator

B body-parser

C cors

D mongoose

express-validator provides easy validation including custom rules.

What happens if a custom validation function returns true?

A Validation is skipped

B Validation fails

C Validation passes

D Server crashes

Returning true means the input passed the custom validation.

Explain how to create and use a custom validation rule in Express with express-validator.

Think about how you tell express-validator to run your own check.

Why are custom validation rules important in web applications using Express?

Consider what built-in validators might miss.