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Page Context "Service"

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Setting a global variable in JavaScript and then reading it from inside our Vue app is, really, a nice way to communicate information from our server to our front-end app. But global variables are still global variables and we should try to at least isolate and organize them as much as possible. Because, for example, what if we changed our app to use the Vue router? Instead of full page refreshes where we set the currentCategoryId as a global variable in Twig, now that data would be returned in a different way: via an Ajax call.

The point is: the way we get currentCategoryId could change. And if we have window.currentCategoryId sprinkled around our code everywhere, it's... not ideal. No problem! Let's isolate our our global variable into a central spot. Enter JavaScript services!

JavaScript Services

I think you're really going to like this. Inside my js/ directory, create a new folder called services/. So far, everything we've worked on has been Vue components... but there's a lot more to our app. We have code for making Ajax calls - which we will eventually centralize - and we're also going to have generic logic that we want to reuse from multiple places.

In our app, the services/ directory is going to hold files that help us fetch data. So, it's a bit different than services in Symfony, which are any classes that do work. By services here, I mean API services... though you could also fetch data from local storage... or even by reading a global variable that we set in the template! Those are all sources of data.

Creating the page-context service

So inside services/, create a new file called page-context.js. I totally invented that name: the purpose of this file will be to help read data related to what "page" we're on - like the current category id.

Inside, instead of exporting a default, like we've been doing so far with Vue, we're going to export named functions. As we start adding more functions to this file, you'll see how we can use whichever one or two functions we need. Say export function, call it getCurrentCategoryId and, inside, very simply, return window.currentCategoryId!

* Returns the current category id that's set by the server.
* @returns {string|null}
export function getCurrentCategoryId() {
return window.currentCategoryId;

Just like that! We have a central place to read our global variable! I'll celebrate by adding some JSDoc above this.

Thanks to this, if we get this information some other way in the future, we will only need to update code in this one spot. I love that!

Now, in sidebar.vue, we can use this like any normal JavaScript module. But I'm going to type this import a little backwards: import from '@/services/page-context'. I left the part after import blank, which is totally not valid JavaScript. But now, I can add {} and autocomplete the getCurrentCategoryId function.

96 lines | assets/js/components/sidebar.vue
// ... lines 1 - 48
// ... line 50
import { getCurrentCategoryId } from '@/services/page-context';
// ... lines 52 - 76
// ... lines 78 - 96

Nice! Down in the computed property, use this: return getCurrentCategoryId().

96 lines | assets/js/components/sidebar.vue
// ... lines 1 - 52
export default {
// ... lines 54 - 65
computed: {
currentCategoryId() {
return getCurrentCategoryId();
// ... lines 71 - 75
// ... lines 77 - 96

That is lovely! When we move over and refresh... it works!

Loading currentCategoryId in the Correct Component

I'm really happy that we've centralized this global variable into a shiny new module. But I want to do just a little bit of future proofing. In a real app, you may or may not choose to do this - but it'll be a good exercise and it will help us later.

Here we go: currentCategoryId is not something that will change while our app is running. Because, when we click on a different category, the page refreshes and the entire Vue app restarts. For the duration of our page view, currentCategoryId never changes!

This means that it isn't something that needs to live in data: we don't need anything to re-render when it changes. That's why it's totally legal to grab this value in sidebar.vue or anywhere else that needs it.

But I want to kind of future proof our app... and plan ahead for a future where currentCategoryId will change while my Vue app is running. If you pretend that currentCategoryId did need to be in data, what component would that data live in?

Remember: the answer to this question is always find the deepest component that needs the data. If I look in products.vue, we know that sidebar needs to know the currentCategoryId so that it can highlight that category. And catalog is also going to need to know the current category soon so that we can print the category title and filter the product list to show only those in that category.

This means that if currentCategoryId were data, it would need to live on the products components so that we could passed it down into sidebar and catalog as a prop.

Replace currentCategoryId Computed with a Prop

Now, I don't actually want to turn the currentCategoryId into data right now because... I don't need to. But I do want to structure my app with this in mind. To start, copy the computed property from sidebar.vue, and, in products.vue add it there.

52 lines | assets/js/pages/products.vue
// ... lines 1 - 17
// ... lines 19 - 20
import { getCurrentCategoryId } from '@/services/page-context';
// ... line 22
export default {
// ... lines 24 - 33
computed: {
// ... lines 35 - 40
currentCategoryId() {
return getCurrentCategoryId();
// ... lines 45 - 49

Oh, and this is cool! When I pasted, check it out! It added the import for me automagically! It did mess up the code style, but you can fix that in PhpStorm if you want. That's better.

So, instead of having currentCategoryId as data, we will have it as a computed property... but inside the component where it would, in theory, need to live as data. That will make it super easy to change to data later if we need to.

Now, pass this to sidebar with :current-category-id="currentCategoryId".

53 lines | assets/js/pages/products.vue
// ... lines 2 - 4
// ... line 6
// ... line 8
// ... lines 10 - 16
// ... lines 18 - 53

And in sidebar.vue, instead of a computed property, we'll set this as a prop. Add currentCategoryId with type String - this is the IRI string - and also default: null.

94 lines | assets/js/components/sidebar.vue
// ... lines 1 - 48
// ... lines 50 - 51
export default {
// ... line 53
props: {
// ... lines 55 - 58
currentCategoryId: {
type: String,
default: null,
// ... lines 64 - 73
// ... lines 76 - 94

The reason I'm using default null is that this will allow the prop to be a String or null, which is what it will be on the homepage. You can add more customized prop validation if you want... but this is good enough for me!

If you scroll down, our currentCategoryId computed property is angry! It says duplicate key currentCategoryId because we don't want to have this as a prop and also as a computed prop. Delete the computed property... and we can also delete the import to celebrate. Our code is happy!

Check it in the browser!

Moment of truth! When we move over... yes! It's still working. Yay for not breaking things!

If you're not sure why we did this, here's what's going on. By moving currentCategoryId up to products.vue and passing it as a prop to sidebar, it would now be very easy for us to change the currentCategoryId computed prop into data. In fact, if we did that, everything else would... well... magically not break!

Next, let's get to work on the catalog component. Let's pass currentCategoryId as a prop so we can filter the list of products to only those for that category.