watch + Install jQuery with Yarn

The downside of using something like Webpack - or any tool that builds your assets - is that every time you make a change, you have to re-run webpack. That's lame.

Fortunately, like many tools, Webpack has a "watch mode", and it just works. Right now, if I click delete, it says, "Delete this log?"

Re-run webpack, but this time with a --watch flag:

./node_modules/.bin/webpack --watch

It still builds the assets... but then waits. It's now constantly watching all your files for changes. Creepy.

Put it to the test: in RepLogApp.js, add a few more question marks to the delete modal for emphasis and save:

Refresh the page and.... we instantly see the change.

This is nothing earth-shattering... it's just a tool we need... and Webpack provides it without any setup. In the terminal, you can even see that it re-dumped our file.

There is one caveat with watch: and we'll see it a few times. If you make a change to the webpack.config.js file itself, you will need to manually restart webpack. Webpack watch does not detect changes to itself.

The Classic Problem with JS Dependencies

Ok, it's time to do something completely different... and unlock a massively powerful new tool. I'm going to say that a lot in this tutorial... and I mean it every time!

Right now, RepLogApp only works because jQuery is available globally:

We have a self-executing function, which expects jQuery to be a global variable by the time this file is loaded. That works because, in our base layout - app/Resources/views/base.html.twig - we include jQuery:

This gives us two global variables - $ and jQuery - which we use everywhere else.

And this is the big, huge, classic, horrifying problem we want to fix: when we write JavaScript, our code is not self-contained. Nope, we need to very carefully make sure to remember to include any dependent script tags before loading our JavaScript. If you forget to add a script tag or add it in the wrong order... bam! Your user sees an error. And you see your face hitting your palm.

This is a terrible and unprofessional way to live. But with Webpack, we can finally fix this. Instead of crossing your fingers and hoping that jQuery is included on this page, in RepLogApp.js, we will require it.

Installing jQuery via Yarn

In our terminal, we already used yarn earlier to install webpack. But we can also use it to install other, front-end libraries... like jQuery. Try it:

yarn add jquery --dev

And just like that! jQuery now lives inside node_modules/:

That's great! Because we can now require it like any other module.

At the top of RepLogApp.js, add const $ = require('jquery'):

For simplicity, we can remove the $ from the self-executing function and the jQuery argument at the bottom:

Now, we require jQuery from node_modules and assign it to the $ variable. Whenever we reference $ in this file, that required value is used. We are no longer dependent on whether or not a global $ or jQuery variable exists.

What Happens when you Require a Module

There are two very important things I want to point out. First, we just said require('jquery'):

We now know that because this does not start with ., Node knows to look for a core library - like it did with path - or to look inside node_modules/, which is what happens this time. That's perfect!

The second very, very, very important thing is that jQuery acts differently, depending on whether or not it's being included with a traditional script tag - like in our layout - or if it's being required by a module system. This is a very common thing you'll see.

Let me show you: I'll hold command to click into the jquery module itself. Yea, it's a bit hard to read. But, on top, it checks to see if typeof module === "object", and typeof module.exports === "object". Basically jQuery is checking whether or not it is being used inside of a module environment, called a commonjs environment.

If it is, it does not create global jQuery or $ variables. Nope, instead it uses module.exports to export the jQuery function. This is really important: it means that when you require jquery, it returns the jQuery function... but does not actually create a global variable. This is a pivotal difference between classic JavaScript and modern JavaScript: instead of creating and using global variables, we require modules and export things from these modules.

Phew! Let's try this out already! In my terminal, I'll check out my watch tab. Yep, it's still working, and should have already detected our new require() and dumped the new file.

Back in the browser, refresh! Yes! Everything still works! It's not super obvious yet, but RepLogApp is now using the required jQuery instead of the global script tag.

But, I can't remove that script tag from the base layout yet... because we have other pages that depend on it. And yes, that means that - until we fix this - our users are downloading jQuery twice: once in the base layout and again when they download rep_log.js.

Open up that file: web/build/rep_log.js. Yep, most of this file is now jQuery.

Now that we have this super power, let's repeat it with SweetAlert... and discover one lingering issue.