res.render passing in object vs res.render.locals variables?

Hey Nick, I think probably at the end of the day it just comes down to preference. That being said there are some difference and definitely some use cases where you would want to res.locals over just passing in an object to res. render().

The properties added to the res.locals object persist through the entire request/response cycle meaning you could assign a value in a middleware function somewhere and then access it in another middleware function later or in PUG. If you haven't talked about middleware functions yet this may not make that must sense to you but if you have, let's look at this example.

File Structure:

views/example.pug

app.js

app.js Javascript File

const express = require('express');
const app = express();

app.set('view engine', 'pug');

//Route for 'example'
//Call the middleware function first, then go to my anonymous function
app.use('/example', middleware, (req, res, next)=>{
    console.log(res.locals.secret);
    return res.render('example', {
        prompt: "Hello World!"
    })
})

//Define a middleware function
function middleware(req, res ,next){
    res.locals.secret = 'SuperSecret';
    return next();
}

app.listen(3000, ()=>{
    console.log("listening on port 3000");
});

example.pug pug file

html
    body
        h1= secret
        h1= prompt

console output

listening on port 3000
SuperSecret

Let's walk through this now. 1) We define a middleware function called 'middleware' which assigns the value 'SuperSecret' to the secret property of the res.locals object. The function then calls the next() method telling express we're ready to move to the next middleware function, in this case our anonymous function. 2) After the first middleware function completes ( next() ) our anonymous function gets called. In this function we print to the console the value res.locals.secret which was defined in our middleware function. We then render our example pug file. 3) In our pug file we have two headers, one that prints our prompt ('Hello World!') and the other prints the value secret which was defined in our first middleware function.

So in conclusion, we can use res.locals to "pass" values between our middleware functions and sort of "append" to the res.locals object as we move through the request/response cycle. Also notice that we can define a value in one function and render a pug template in another and PUG still can access that res.locals value. Pretty Cool!