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Passing Data from your Server to React

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Look inside RepLogCreator. The items in the drop-down are hardcoded. But, in reality, we can't just put whatever we want here: there is a specific set of valid options stored in our backend code.

We already know this is true because the last option is totally fake! When we send that to the server, it hits us with a validation error.

So, here is the question: instead of hardcoding these options, should we load them dynamically from the server?

The answer is... maybe? If these options won't ever change or change often, it's really not that big of a deal. The advantage is... simplicity!

But, if they will change often, or if having an invalid one on accident would cause a hugely critical or embarrassing bug, then yea, you should load them dynamically... so that you can sleep soundly at night.

Refactoring Data to the Top Level Component

Whenever your JavaScript app needs server data, there are two options. But, they both start the same way: by moving our itemOptions up into RepLogApp, which is the only component that's even aware a server exists!

Copy itemOptions and then open RepLogApp. On top, initialize a new itemOptions state set to that array.

157 lines | assets/js/RepLog/RepLogApp.js
// ... lines 1 - 7
constructor(props) {
// ... lines 9 - 10
this.state = {
// ... lines 12 - 18
itemOptions: [
{id: 'cat', text: 'Cat'},
{id: 'fat_cat', text: 'Big Fat Cat'},
{id: 'laptop', text: 'My Laptop'},
{id: 'coffee_cup', text: 'Coffee Cup'},
{id: 'invalid_item', text: 'Dark Matter'}
// ... lines 27 - 32
// ... lines 34 - 157

Because all state is automatically passed as props to RepLogs, go there and add the new prop type: itemOptions as an array that is required.

115 lines | assets/js/RepLog/RepLogs.js
// ... lines 1 - 99
RepLogs.propTypes = {
// ... lines 101 - 112
itemOptions: PropTypes.array.isRequired

Above, destructure that, then, below, pass it down to RepLogCreator as itemOptions={itemOptions}.

115 lines | assets/js/RepLog/RepLogs.js
// ... lines 1 - 17
export default function RepLogs(props) {
const {
// ... lines 20 - 31
} = props;
// ... lines 34 - 39
return (
// ... lines 41 - 88
// ... lines 90 - 91
// ... lines 94 - 96
// ... lines 99 - 115

Copy the prop type, then do the same in RepLogCreator: define the prop type at the bottom, then go to the top of the function to destructure out itemOptions.

101 lines | assets/js/RepLog/RepLogCreator.js
// ... lines 1 - 3
export default class RepLogCreator extends Component {
// ... lines 5 - 45
render() {
// ... line 47
const { validationErrorMessage, itemOptions } = this.props;
// ... lines 49 - 92
// ... line 95
RepLogCreator.propTypes = {
// ... lines 97 - 98
itemOptions: PropTypes.array.isRequired

Below, use the local itemOptions variable for the map function.

101 lines | assets/js/RepLog/RepLogCreator.js
// ... lines 1 - 45
render() {
// ... lines 47 - 49
return (
// ... lines 51 - 68
{ => {
// ... lines 70 - 91
// ... lines 94 - 101

When we refresh... cool! The options aren't dynamic yet, but they are stored as state. If you change a value... yep! It shows up.

Two Ways to Load Server Data

Now that the data lives in our top-level component, let's talk about the two ways we can load this dynamically from the server. Actually, we already know the first way - we did it with repLogs! We could set itemOptions to an empty array, then make an AJAX call from inside componentDidMount(). Of course, we would also need to create an API endpoint, but that's no big deal.

Or, you could use the second option: render a global variable inside Twig and read it in JavaScript. The advantage is that this data is available immediately: you can populate your app with some initial data, without waiting for the AJAX call.

Passing the Options as Props

Copy the options again and go into the entry file: rep_log_react.js. This will not be the final home for these options - but it will get us one step closer. Create a new itemOptions variable and paste! Now, pass these as a new prop: itemOptions={itemOptions}.

22 lines | assets/js/rep_log_react.js
// ... lines 1 - 6
const itemOptions = [
{id: 'cat', text: 'Cat'},
{id: 'fat_cat', text: 'Big Fat Cat'},
{id: 'laptop', text: 'My Laptop'},
{id: 'coffee_cup', text: 'Coffee Cup'},
{id: 'invalid_item', text: 'Dark Matter'}
// ... line 14
// ... line 17
// ... line 20

Thanks to this, RepLogApp will now receive a new itemOptions prop. Remove the state entirely.

At the bottom, set this prop type: itemOptions is an array, and you could make it required - I'll talk more about that in a minute.

152 lines | assets/js/RepLog/RepLogApp.js
// ... lines 1 - 147
RepLogApp.propTypes = {
// ... line 149
itemOptions: PropTypes.array,

Oh, and this is cool! We deleted the itemOptions state but added an itemOptions prop. And because we're passing all props & state to RepLogs, it is still receiving an itemOptions prop. In other words, this just works.

Side note: I originally set itemOptions to state because this is needed if you wanted to make an AJAX call to populate them: they would be empty at first, then change a moment later when the request finished. But really, itemOptions don't ever need to change. So once we passed them as props, we could remove the state.

But, if the item options really did need to be state - if this was something that changed throughout the life of our app - we could still use this strategy. We could use the itemOptions prop to set the initial value of the state. This literally means that you would still have an itemOptions state, and it would be initialized to this.props.itemOptions.

I might even call the prop initialItemOptions for clarity... though if you do have a state and prop with the same name, that's fine. If you look down in render(), the state would override the prop, because the ...state comes second.

Setting Initial Props

Anyways, down in propTypes, I did not make itemOptions a required prop. In a real application, I probably would: I don't want the select to ever be empty. But sometimes, you will create a component where you want a prop to be truly optional. And in those cases, you need to be careful: if we didn't pass the itemOptions prop, our code would explode! itemOptions would be undefined instead of an array... which would be a problem when RepLogCreator calls .map on it.

To solve this, you can give any prop a default value. It's super easy: add RepLogApp.defaultProps = an object with itemOptions set to an empty array.

155 lines | assets/js/RepLog/RepLogApp.js
// ... lines 1 - 152
RepLogApp.defaultProps = {
itemOptions: []

Ok: we have removed the hardcoded itemOptions from our React app entirely. But... we're not done: they're still hardcoded in rep_log_react.js. We need to fetch this value dynamically from the server. Let's do that next!