What does localhost:1337 become in production? - node.js

Question

I'm sure this question is very basic but it's confusing me.
Currently in development mode my backend runs in localhost:1337 (using Strapi cms to be precise).
But what will it turn into when the website is deployed to a web server?
Specifically, the website will reside in a sub-directory and not in the root, will it affect this?
Currently this is a url I'm using inside of my project as images src:
localhost:1337/uploads/image-example.jpg
I'm trying to understnad what will be the new url once the project has been deployed, and what's more it's deployed in a subdirectory and not in the root
Thanks in advance

Answer 1

You shouldn't have to do anything.
Most, if not all, web hosts use the PORT environment variable to configure the application server. I just did a quick code search through Strapi and found this:
"port": "${process.env.PORT || 1337}",
to read the environment variable or use a default value (instead of asking for a random port with 0, for predictability while developing). If you used their starter script, I would assume your application has the same.
In many cases, this is sufficient, with two exceptions.
If your application server doesn't know its URL, it can only generate relative URLs. For example, it won't be able to compose an email containing a link to itself.
HTTP redirects required an absolute URL once upon a time, but that has since been amended.
Setting just the port assumes that the application will be mounted on /. If, for example, you were configuring your own web server (instead of Heroku), and you wished to mount your application on /blog, you would need some way to tell your application so that it can put the right prefix on paths.
It might be possible to get around this by restricting the URLs to relative paths only, but that sounds tedious and I don't think it's usually done. And an incoming request doesn't actually have enough information to discern the URL that the user is looking at. Instead, you show the URL to your app out of band.
For example, Grafana has a root_url variable in its config file where you put the full URL prefix, Prometheus takes a -web.external-url argument, etc.
However, I don't think Heroku can mount your application anywhere other than /, so the port should be sufficient.
Here's a complete server that runs on Heroku and shows its environment to you:
const app = require("express")();
app.set("json spaces", 2);
app.get("/", (req, res) => {
res.send(process.env);
});
const server = app.listen(process.env.PORT, () => {
console.log('server is up on', server.address());
});
The PORT variable is all the configuration it needs; the environment does not include the app name.
Hardcoding the production URL of your app can become a maintenance burden, so make sure it remains configurable with a command line argument or environment variable if you do end up teaching your app what its URL is.
So to answer your question specifically, your image src should begin with /uploads.