Business Logic Helpers

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We've already been organizing our code in several ways. The biggest way is that we've been breaking our components down into smaller pieces, which is awesome! We also created services for fetching data, whether that's via AJAX calls or by grabbing a global variable.

But these aren't the only ways we can organize our code. In JavaScript, like with most languages, if you have a chunk of code that's complex or that you want to reuse, you can totally isolate that into its own file. This is exactly what we often do in Symfony with service classes.

Why Isolate Logic?

Look at product-card.vue. We created a computed property that takes the product's price, divides it by a hundred and then converts it into decimal digits to display in the template.

Having logic inside your components is a bit like having logic inside of controllers in Symfony. It's not the worst thing ever, but it makes your controllers harder to read. It also means that you can't reuse that code from other parts of your app or unit test it.

The same is true in JavaScript. By isolating logic like this into a separate file, we can keep our components readable, re-use logic and, if you want, unit test it.

But... I'm not going to put this logic into the services/ directory because, at least in this project, I'm using services/ to mean "things that fetch data". In functional programming, a helper is a term that's often used for functions that take input, process it and return something else. And this is exactly what our new function will do.

Create a Helper Function

So, inside of js/, create a new directory called helpers/ and then a new file called format-price.js. In here, export default and, actually, let's use the more hipster arrow syntax to say that I export default a function.

For the body of that function, go to product-card, copy the formatting code and... paste. But change to use the price argument.

Brilliant! Now, you might notice that ESLint is angry. Does it NOT like hipster code? How dare you, ESLint? Oh, no! Phew...! It says

Unexpected block statement surrounding arrow body. Move the return value immediately after the arrow.

My ESLint rules are set up so that if I have an arrow function that only has one line, and that one line a return statement, we should add parentheses around the statement then remove the return and the semi-colon at the end.

That's now an implied return.

If you don't like that, just use the normal function syntax. And, to earn the admiration of our teammates, let's add some JSDoc: the price is a number and let's even describe what the function does.

And... woo! We now have a nice reusable function! Oh, and there are two ways to organize your helpers... or JavaScript modules in general. First, you can have a file, like format-price, which exports default a single function. Or, if you have several different helper functions related to pricing or number manipulations, you could create a file called, maybe, number.js and then export named functions. That second idea is what we're doing inside of services/. It's up to you to decide which you like better.

Using the Helper Function the Wrong Way

Ok! Let's go use this inside of product-card.vue. The first thing we need to do is import it into the component. Do that with import formatPrice from '@/helpers/format-price'

Now, when I first started using Vue, I thought:

Hey! I now have a local variable called formatPrice in this file! So let's go right up to the template and use it! formatPrice() with product.price to reference the product object and its price property.

But... it was not to be! If you move over to the browser's console... our dreams are crushed! It says:

Property or method formatPrice is not defined on the instance, but referenced during render.

Of course! When you reference a variable or function in a template we know that what it really does is call this.formatPrice(). It does not try to find some local formatPrice variable. On our instance, we do have a computed property called price but no methods.

Using the Helper in our Component code

So... we can't just import formatPrice and expect it to magically be available in the template. But we can use it in our JavaScript code, like in our computed property.

Change the template code back to price. Now, in the computed method, use the new helper: return formatPrice(), and pass the same thing we did before: this.product.price.

This time, when we move over... yes! It works perfectly! Let me get rid of the search term and... nice! No errors in the console.

Next: Let's make our search bar a little bit fancier by adding an X icon on the right to clear the search!

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