Architecture

How express-validator works

In express.js terminology, middlewares are functions that receive the request (req) and response (res) objects of an incoming HTTP request. Middlewares can respond to the request, or pass control over to the next registered middleware.

express-validator is implemented mostly as a bunch of middleware functions, which will read the request and verify if it's valid or not according to the specification you declared for each field. Any invalid fields are then recorded internally, and validation errors can be forwarded back to the user for them to fix up their request and try again.

Since it comes with many built-in validators and sanitizers, the style of specifying the validations as middlewares confers express-validator a somewhat declarative programming style. You usually don't tell it when the validations are meant to run, though you can have control over that if you wish.