This is straight out of the express-validator documentation. I noticed that when these functions are passed as middleware, they include arguments and parenthesis, in which case they should be called at runtime right?
// ...rest of the initial code omitted for simplicity.
const { body, validationResult } = require('express-validator');
app.post(
'/user',
// username must be an email
body('username').isEmail(),
// password must be at least 5 chars long
body('password').isLength({ min: 5 }),
(req, res) => {
// Finds the validation errors in this request and wraps them in an object with handy functions
const errors = validationResult(req);
if (!errors.isEmpty()) {
return res.status(400).json({ errors: errors.array() });
}
User.create({
username: req.body.username,
password: req.body.password,
}).then(user => res.json(user));
},
);
I jumped into the source code to try and figure out how they are preventing the function calls, but it is a little over my head. The reason I wanted to learn about this was I was interested in creating a middleware that worked in a similar fashion, where arguments could be passed without actually calling the function at runtime.
I'm not going to reverse engineer some specific code, but will explain how to achieve this in general.
See the documentation for middleware for reference.
An endpoint is a function that takes two arguments. The request and the response. They are typically named req and res.
Middleware takes three arguments. The third is next which is called to pass control to the next function.
app.use(function (req, res, next) { console.log('Time:', Date.now()) next() })
Now, middleware doesn't have to pass control to the next function. It can just respond.
const middleware = (req, res, next) => {
if (typeof req.body?.username === 'undefined') {
// No username was provided
res.send("Error: No username was provided");
} else {
next();
}
}
Now you might want this to be reusable for arguments other than username, so you can write a factory function which returns the middleware function.
const createMiddleware = (propertyName) => {
const middleware = (req, res, next) => {
if (typeof req.body?.[propertyName] === 'undefined') {
// No value was provided for the propertyName
res.send(`Error: No ${propertyName} was provided`);
} else {
next();
}
}
return middleware;
}
And then use it:
app.use( createMiddleware('username') );
app.use( createMiddleware('password') );