No Data Duplication! Fancy Computed Prop

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This cool new completeItems array combines data from two AJAX calls: one for the cart and another for the products that are in that cart. If we could make this available to our template, we could loop over it and start printing out real product data.

... lines 1 - 28
<script>
... lines 30 - 34
export default {
name: 'ShoppingCart',
... lines 37 - 41
watch: {
async cart() {
... lines 44 - 48
const completeItems = this.cart.items.map((cartItem) => (
{
product: products.find((product) => product['@id'] === cartItem.product),
color: cartItem.color,
quantity: cartItem.quantity,
}
));
console.log(completeItems);
},
},
};
</script>
... lines 61 - 71

So... how can we do that? Easy! Create a new completeItems data, set it here, then reference it in the template! We're unstoppable!

Be Careful with Duplicate Data

But... there's a problem with that approach, and I bet some of you see it too. What is it? Duplicated data. Duh, duh, duh!

If we set completeItems onto data, then some of the basic cart item data - the product IRI, color IRI and quantity - would be duplicated in two places. They would be reflected in completeItems... but they would also be on the cart data itself.

And we never want to store a piece of data in multiple places. Because, if we later changed a piece of data - like the quantity of an item - it would change in one spot... but not the other... unless we added extra code to keep them in sync. Yuck. It's really no different than a database: you typically don't want to store a piece of data in multiple places because they could get out of sync.

Computed Prop to the Rescue

So... let's be smarter. Think about it: the only new piece of data we have is the products array that we get back from the AJAX call. If we stored that as data... we could still access this nice completeItems array in the template via a computed property.

Let's do it! Start by adding a data key, which is a function, then returning an object with products initialized to null.

... lines 1 - 28
<script>
... lines 30 - 34
export default {
name: 'ShoppingCart',
... lines 37 - 41
data() {
return {
products: null,
};
},
... lines 47 - 63
};
</script>
... lines 66 - 76

Down below in the watcher function, instead of const products, say this.products... and reference this.products below in the loop.

... lines 1 - 28
<script>
... lines 30 - 34
export default {
name: 'ShoppingCart',
... lines 37 - 41
data() {
return {
products: null,
};
},
watch: {
async cart() {
... lines 49 - 51
this.products = productsResponse.data['hydra:member'];
... line 53
const completeItems = this.cart.items.map((cartItem) => (
{
product: this.products.find((product) => product['@id'] === cartItem.product),
... lines 57 - 58
}
));
console.log(completeItems);
},
},
};
</script>
... lines 66 - 76

Next, add a computed key with one new computed prop. Call it completeCart().

... lines 1 - 28
<script>
... lines 30 - 34
export default {
name: 'ShoppingCart',
... lines 37 - 46
computed: {
completeCart() {
... lines 49 - 63
},
},
... lines 66 - 73
};
</script>
... lines 76 - 86

Before we do anything else in this function, if someone calls us and the cart data is not ready yet, we should also return null. So if !this.cart... or if !this.products - if we also haven't finished loading the products - then return null. This means that if completeCart returns null, things are still loading.

... lines 1 - 28
<script>
... lines 30 - 34
export default {
name: 'ShoppingCart',
... lines 37 - 46
computed: {
completeCart() {
if (!this.cart || !this.products) {
return null;
}
... lines 52 - 63
},
},
... lines 66 - 73
};
</script>
... lines 76 - 86

Now, copy all this completeItems stuff from watch, move it here, but instead of logging, return a new object. We'll make this look just like the cart... just for consistency: with an items key set to completeItems.

... lines 1 - 28
<script>
... lines 30 - 34
export default {
name: 'ShoppingCart',
... lines 37 - 46
computed: {
completeCart() {
if (!this.cart || !this.products) {
return null;
}
const completeItems = this.cart.items.map((cartItem) => (
{
product: this.products.find((product) => product['@id'] === cartItem.product),
color: cartItem.color,
quantity: cartItem.quantity,
}
));
return {
items: completeItems,
};
},
},
... lines 66 - 73
};
</script>
... lines 76 - 86

That should do it! If we've done everything correctly, after the cart data has finished loading, the watcher will call our function, we will make an AJAX request for the products, and then, finally, in our template, we will reference the completeCart variable, which will combine all that data once it's available. Remember: one of the magic things about computed properties is that Vue will automatically re-render and re-call our computed function whenever any piece of data that it references - like this.cart or this.products - changes.

Rendering Complete Cart Data

So let's go to our template. We basically want to update everything from cart to completeCart. Copy that, use it on the v-if and inside the v-for.

Then, since cartItem.product will now be an object, we can prove everything works by printing cartItem.product.name. Oh, and I'll change one more spot to completeCart.

<template>
<div :class="[$style.component, 'container-fluid']">
<div class="row">
... lines 4 - 5
<div class="col-xs-12 col-lg-9">
... lines 7 - 8
<div class="content p-3">
<loading v-show="completeCart === null" />
<div v-if="completeCart !== null">
<div
v-for="(cartItem, index) in completeCart.items"
:key="index"
>
{{ cartItem.product.name }} ({{ cartItem.quantity }})
</div>
<div v-if="completeCart.items.length === 0">
Your cart is empty! Get to shopping!
</div>
</div>
</div>
</div>
</div>
</div>
</template>
... lines 28 - 86

Testing time! Back at the browser... ha! I didn't even need to refresh! It's already printing the product name. I know, it doesn't look that impressive yet, but we did just put together a lot of data.

Check out the Vue dev tools for this component: cart data, products data and a beautiful completeCart computed prop that allows us to easily use the data we need in the template, without duplicating anything.

Next: we're still missing one piece of data inside completeCart: the color. This is still an IRI string... but what we need is the color data, which will include the hex color so that we can render a color box on the screen. Let's finish that and learn a cool trick for making AJAX requests in parallel.

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What JavaScript libraries does this tutorial use?

// package.json
{
    "devDependencies": {
        "@fortawesome/fontawesome-free": "^5.15.1", // 5.15.1
        "@symfony/webpack-encore": "^0.30.0", // 0.30.2
        "axios": "^0.19.2", // 0.19.2
        "bootstrap": "^4.4.1", // 4.5.3
        "core-js": "^3.0.0", // 3.6.5
        "eslint": "^6.7.2", // 6.8.0
        "eslint-config-airbnb-base": "^14.0.0", // 14.2.0
        "eslint-plugin-import": "^2.19.1", // 2.22.1
        "eslint-plugin-vue": "^6.0.1", // 6.2.2
        "regenerator-runtime": "^0.13.2", // 0.13.7
        "sass": "^1.29.0", // 1.29.0
        "sass-loader": "^8.0.0", // 8.0.2
        "vue": "^2.6.11", // 2.6.12
        "vue-loader": "^15.9.1", // 15.9.4
        "vue-template-compiler": "^2.6.11", // 2.6.12
        "webpack-notifier": "^1.6.0" // 1.8.0
    }
}