Computed Properties

This special class syntax gave us the power to conditionally add the collapsed class. This works nicely, but it is kind of a lot of logic to have in our template. It's gettin' a bit ugly. A better solution might be to calculate which classes this component should have using JavaScript and then pass that value into our template as a variable.

Whenever you need to calculate a value, like an array of classes, based off some props or data, the way to do that is with a computed property. Here's how it works: Down inside our JavaScript code, right below data, add a new option called computed and set it to an object.

Just like with methods down here, we'll populate this with functions. Add our first computed property called componentClass() - you can name that anything. This will return the array of classes that our component should have. But don't worry about how we're going to use it yet: let's just start by filling in the logic. First we'll say const classes = and set that to an array with the three classes that we always need. this.$style.component (we'll talk about that in a second), p-3, and mb-5. Perfect!

At the bottom, return classes. We'll worry about the conditional class in a minute.

.this magic

But let's talk about this.$style.component real quick:

We know that as soon as we add module to our style tag, Vue makes a new $style variable available in our template. We use that to say things like $style.component.

A few minutes ago, we learned that anytime you reference a variable inside of a template, internally, what that actually does is call this.$style. We just don't have to say this. in the template because Vue adds it for us automatically.

So even if we knew nothing else, the very fact that we can reference the $style variable in a template means that the vue instance must have a $style property on it. In other words, we are allowed to say this.$style inside of JavaScript. That's why this works in our computed property method.

For the conditional class logic, let's say if this.collapsed, to reference our collapsed state, then classes.push(this.$style.collapsed).

That's it! And of course, with any methods, adding some documentation is always nice.

Here, PhpStorm tries to guess the return type... but gets a little confused since Vue is so dynamic. Let's help it: this returns an array of strings.

Excellent!

How computed works

So here's the deal. As soon as you have a key under the computed option, it becomes available in the template as a variable.

Copy componentClass and, up in the template, very simply, we'll say :class="componentClass".

Up until now, we know that Vue adds all keys under data, props and methods to the Vue instance, which means that we can reference those inside our template. Well, computed is the fourth and final thing that gets added to the instance. Vue adds each key under computed as a property.

This means that, up in the template, we can just reference componentClass, which is really this.componentClass. But behind the scenes, when we access that property, Vue will actually call the componentClass function to get it. It's able to do that thanks to the fake getter property trick that we saw earlier when we console.logged the this variable.

And really, the only difference between methods and computed is the syntax: we use methods like methods and computed like properties. Oh, and also, Vue caches computed properties so that it only needs to call our function when something actually changes.

Anyways, because computed keys are added to the Vue instance, just like methods, data or props, it means that we can reference them with the this variable. To prove it, inside of our toggleCollapsed method, let's say console.log(this.componentClass).

Notice that PhpStorm tries to autocomplete that with parentheses, but that's not right! We need to reference it like a property, even though we know that Vue will call our method.

Check it out!

So if we go over to the browser now... check this out! You can see the log and it shows the component classes correctly every time we change it!

You can also see this over in the Vue dev tools! Click down on Sidebar. Under data, you now have a computed section with componentClasses. This changes when you hide and show the sidebar!

Back at our editor, remove the console.log(). We just mastered one of the most powerful tools in Vue: computed properties! Nice work!

Next, what happens when we need to access a piece of data - like collapsed - in a different component? If that component is a child, we can pass it down as a prop. But... what if it's not?