Where should a Piece of Data Live?

This collapsing sidebar looks better... except that the layout doesn't change. Hmm, it would make more sense if the main content moved over to take up more space.

Right click and inspect element on this area. When we collapse the sidebar, what we need to do is change the classes on the <aside> to take up less space and change the classes on the main <div> to take up more space.

Pff. No problem: we just need to make a few classes dynamic!

Yes... but... it's not so simple. The collapsed data lives inside of sidebar. But the elements where we need to use that information do not. They live inside products.vue: here's the <aside> and this is the <div>. Somehow we need to access the collapsed data right here.

Never Duplicate Data

Let me first say one important thing: a piece of data - like collapsed - should never be duplicated. It must always live in exactly one spot. So what we're not going to do is create another collapsed data inside of products... and then - somehow - try to keep the new collapsed data in sync with the collapsed data in sidebar. Yuck!

Nope, a piece of data should always live in exactly one spot.

Now, if you use something like Vuex - or some strategies in Vue 3 that we'll talk about in a future tutorial - then it's possible to store your data in a central location, outside of your components. Then multiple components can get and set that data directly. But in our case, there's only one place that data can live: inside a component.

In which Component should Data Live?

And so, each time you need to introduce a new piece of data into your app, there's a natural question: which component should this data live in? Like, why did we put collapsed in sidebar instead of, maybe inside products.vue?

Here's the rule: you should add data to the deepest component that needs it. By deepest, I'm referring to the component hierarchy that you can see in the Vue Dev tools: Sidebar is a deep component and Products is higher.

Now, until this moment, the only component that needed access to the collapsed data was Sidebar. So it made perfect sense to put it inside Sidebar. But now we realize that we also need access to the collapsed data inside of Products.

To make this work, we actually need to move the collapsed data from sidebar up into the products components. That will allow us to pass it down into sidebar as a prop. You can't pass data up, but you can pass it down.

Moving the collapsed Data

Let's get to work! Inside of products.vue, let's add the new data. Create the data() function. Call the data sidebarCollapsed - so we know what is collapsed - and initialize it to false.

Now that the data lives here, we need to pass it to sidebar. No problem! Say :collapsed - because we need this to be set to a dynamic value - ="sidebarCollapsed".

Perfect! To be able to receive this collapsed prop, in sidebar, we need to define it. Add props: set to an object with a collapsed key set to another object. We need type: Boolean and I like adding required: true to make sure it's passed.

We now temporarily have a collapsed prop and a collapsed data and PhpStorm is mad! And... it's right: we can't do that and we don't want to. Delete the collapsed data.

The cool thing is that, because we named the collapsed prop the same as the data we had before, most of our code is just going to work! Vue doesn't care if the collapsed variable is a data or prop.

Let's try it! Find your browser and I'll refresh just to be safe. Click on the Products component in the dev tools and change the new sidebarCollapsed data to true.

Yes! The sidebar collapsed! If you click on Sidebar, you can see that the prop is true. Each time we change the sidebarCollapsed data, the prop in Sidebar also changes.

What Happens when you Modify a Prop

Even clicking the collapse button works! Well... sort of. Click on the console to find... ah! A horrible error!

Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders.

So... here's what's happening. We are able to reference the collapsed prop inside sidebar. But down on the button, when we click it, this calls toggleCollapsed(). Jump down to that method. Ah... then this method changes the collapsed prop.

Earlier, I said that you should never, ever change the value of a prop. Props are meant to be read but not modified. Of course, you might be thinking:

Yeah... but it worked! When we changed the prop, the sidebar totally re-rendered correctly!

And that's true! But not everything is right with the world. Look back at the Vue Dev tools and the Sidebar component. The collapsed prop is false. When we click the button, that correctly changes to true.

But now look at the Products component. Under data, sidebarCollapsed is false! When we click the button it does not change! By modifying the prop, we've cause our sidebarCollapsed data and collapsed prop to get out of sync. If we were using the sidebarCollapsed data in the Products component, the sidebar would probably look half collapsed and half not collapsed.

The point is, each piece of data must live in exactly one location and that location is the "source of truth". That is where the value needs to change.

What we really need to do is somehow have the Sidebar tell the Products component:

Hey! The button was clicked so... the collapsed data should change!

And then the Products component could change its own data. The way to do this is with $emit(). Let's talk about it next.