# Create React App And Mobx

Dec 14th, 2018 - written by Kimserey with .

I have been using state management frameworks for the past few years, mainly with Angular and NGRX. Today we will see how we can get started with Create-React-App using Mobx as a state management system.

## Create a fresh application

Start by installing the latest version of NPM then use npx to setup a fresh project.

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npx create-react-app my-app --typescript


Then navigate to /my-app and run npm start. This will start the application in a development server with a live reload. The command run by npm start is defined under package.json scripts > start and runs react-scripts start.

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cd my-app
npm start


The application should now build and run properly and any changes done on the application should be reflected on the browser. We now have all the necessary tool to start writing code in React.

## Create components

In React with Typescript, creating a component can be done in two ways,

1. using a class
2. creating a function

Using a class, we need to inherit from React.Component and implement the render() function which returns a JSX.Element. While creating a function would just be a function that takes () or props as argument and returns a JSX.Element.

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import React, { Component } from 'react';

export class HelloWorld extends Component {
render() {
return (
<div>Hello World</div>
);
}
}


or as a function:

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const ByeBye = () => {
return (
<div>Bye bye</div>
);
}


Then we can use both of them in App.tsx:

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class App extends Component {
render() {
return (
<div>
<HelloWorld/>
<ByeBye/>
</div>
);
}
}


We can see how we use the component by directly using the component as a tag name <HelloWorld/> or <ByeBye/>. If we want to pass argument from parent component to child component, we use props. In Typescript, we can typesafe the prop by specifying it in the component class Component<TProps>.

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export class HelloWorld extends Component<{ user: string }> {
constructor(props: { user: string }) {
super(props);
}

render() {
return (
<div>Hello World {this.props.user}</div>
);
}
}


Notice that we get the props from the constructor and forward it to the base component super(props). After that we have to specify user on the component in App.

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<HelloWorld user="Kim"/>


React was called react at the first place because it introduces a reactive approach (ok I might have made up that one but I guess that’s the reason). Now if we define a variable in our component:

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class App extends Component {
user = "Kim";

onUserChange = (e: ChangeEvent<HTMLSelectElement>) => {
this.user = e.target.value;
}

render() {
return (
<div>
<select value="Kim" onChange={this.onUserChange}>
<option value="Kim">Kim</option>
<option value="Tom">Tom</option>
<option value="Sam">Sam</option>
</select>
<HelloWorld user={this.user}/>
</div>
);
}
}


We will see that the component only gets rendered once displaying Kim once. The reason is that each component have an internal state. Changing the component state will trigger a rerendering. This state can be access with this.state and can be typesafe as well from Component<any, TState>.

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class App extends Component<any, { user: string }> {
constructor(props: any) {
super(props);

this.state = { user: "Kim" };
}

onUserChange = (e: ChangeEvent<HTMLSelectElement>) => {
this.setState({ user: e.target.value });
}

render() {
return (
<div>
<select value="Kim" onChange={this.onUserChange}>
<option value="Kim">Kim</option>
<option value="Tom">Tom</option>
<option value="Sam">Sam</option>
</select>
<HelloWorld user={this.state.user}/>
</div>
);
}
}


Notice the lambda expression set for the event handler. We do that to capture the context of this to the current class. Here what we do is that we’ve typesafe the state with { user: string } and called setState() changing the user property of the state. When the selection changes, HelloWorld gets rendered with the right value.

We can also go a step further and define our HelloWorld component as PureComponent providing a performance boost. A component can be defined as pure when for the same set of props, it yields the same rendering.

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export class HelloWorld extends PureComponent<{ user: string }> {
constructor(props: { user: string }) {
super(props);
}

render() {
return (
<div>Hello World {this.props.user}</div>
);
}
}


State is great for a single component and simple scenario, but when our application involves a more complex state which needs to be shared across multiple components, we can introduce a state management framework.

## Create an observable state and store

The state management we will be using is Mobx. We start first by installing it.

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npm install mobx --save
npm install mobx-react --save


Next we can create our first state by creating a class in a file AppState.tsx.

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import { observable, computed, action, decorate } from "mobx";

export class AppState {
users = [
"Kim",
"Tom",
"Sam"
];

selectedUser = "Kim";

selectUser(user: string) {
this.selectedUser = user;
}
}

decorate (AppState, {
users: observable,
selectedUser: observable,
selectUser: action
});


We start by importing from mobx observable, action and decorate. We then define a state which is a simple class with users and a selectedUser for variables. And selectUser as action mutating the state.

Lastly we use the mobx functions we imported to call decorate passing in the type of our state AppState, and an object specifying each decorator for our interested variables/properties/actions. Here we specify that our variables are observable meaning that changes occuring on them needs to trigger a rendering. And we specify that the setX are actions which means that they will be mutating the state.

## Create observers components

Now that we have defined the state, we can use it in our application by instantiating it inside our App main component passing it inside the props but to be able to observe the changes of the observable state, we need to decorate our classes as observers. This is done by using òbserver from mobx-react.

For our HelloWorld class, it would be:

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const HelloWorld = observer (
class HelloWorld extends Component<{ store: AppState }> {
constructor(props: { store: AppState }) {
super(props);
}

render() {
return (
<div>Hello World {this.props.store.selectedUser}</div>
);
}
}
);


And for a function component displaying the selected user:

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const SelectedUser = observer ((props: { store: AppState }) => <p>Selected {props.store.selectedUser}</p>);


And the following function to select a user from a option select:

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const SelectUser = observer ((props: { store?: AppState }) => {
const onChange = (e: ChangeEvent<HTMLSelectElement>) => {
props.store!.selectUser(e.target.value);
}

const options =
props.store!.users.map(u => <option key={u} value={u}>{u}</option>);

return (
<select value={props.store!.selectedUser} onChange={onChange}>
{options}
</select>
);
});


We specified that our component expect the state as input therefore we would need to provide the state:

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class App extends Component {
store = new AppState();

render() {
return (
<div>
<SelectedUser store={this.store}/>
<SelectUser store={this.store}/>
<HelloWorld store={this.store}/>
</div>
);
}
}


We now have a state observed by components. Selecting a user from SelectUser would modify the state which would be observed from both SelectedUser and HelloWorld.

But as we see, we are injecting the store on all components. To avoid this repetition, we can use a Provider which would automatically inject the store.

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class App extends Component {
store = new AppState();

render() {
return (
<Provider store={this.store}>
<div>
<SelectedUser/>
<SelectUser/>
<HelloWorld/>
</div>
</Provider>
);
}
}


We start by placing components inside a Provider tag which would define store as a property available in props without the need of specifying it explicitly.

Next on the components where we need the store, we can use inject from mobx-react to inject the store to the props.

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const HelloWorld =  inject("store") (
observer (
class HelloWorld extends Component<{ store?: AppState }> {
constructor(props: { store?: AppState }) {
super(props);
}

render() {
return (
<div>Hello World {this.props.store!.selectedUser}</div>
);
}
}
)
);


Same for the function components:

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const SelectedUser = inject("store") (
observer ((props: { store?: AppState }) => <p>Selected {props.store!.selectedUser}</p>)
);

const SelectUser = inject("store") (
observer ((props: { store?: AppState }) => {
const onChange = (e: ChangeEvent<HTMLSelectElement>) => {
props.store!.selectUser(e.target.value);
}

const options =
props.store!.users.map(u => <option key={u} value={u}>{u}</option>);

return (
<select value={props.store!.selectedUser} onChange={onChange}>
{options}
</select>
);
})
);


Note that we specify the store as an optional variable store? in the definition which makes the type AppState | undefined, therefore to be able to use it, we need to use the non-null-assertion operator props.store!.x which allows us to indicate to Typescript that we know that this value isn’t undefined.

And that’s it, that concludes today’s post!

Here is the full code for reference:

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import React, { Component, ChangeEvent, PureComponent } from 'react';
import { Provider, observer, inject } from 'mobx-react';
import { AppState } from './AppState';

const SelectedUser = inject("store") (
observer ((props: { store?: AppState }) => <p>Selected {props.store!.selectedUser}</p>)
);

const SelectUser = inject("store") (
observer ((props: { store?: AppState }) => {
const onChange = (e: ChangeEvent<HTMLSelectElement>) => {
props.store!.selectUser(e.target.value);
}

const options =
props.store!.users.map(u => <option key={u} value={u}>{u}</option>);

return (
<select value={props.store!.selectedUser} onChange={onChange}>
{options}
</select>
);
})
);

const HelloWorld = inject("store") (
observer (
class HelloWorld extends Component<{ store?: AppState }> {
constructor(props: { store?: AppState }) {
super(props);
}

render() {
return (
<div>Hello World {this.props.store!.selectedUser}</div>
);
}
}
)
);

class App extends Component {
store = new AppState();

render() {
return (
<Provider store={this.store}>
<div>
<SelectedUser/>
<SelectUser/>
<HelloWorld/>
</div>
</Provider>
);
}
}

export default App;


## Conclusion

Today we saw how to create a simple React application using Create React App with Mobx, a state management library, in Typescript. We started by bootstrapping the project and installing the necessary libraries. Next we create our first components and saw two different formats, one as a function and the other one as a class. We then moved on to look into creating a global state managed with Mobx and saw how to define containers which are state observer components. Hope you liked this post, see you on the next one!

Designed, built and maintained by Kimserey Lam.