React

React is a front end web framework built entirely out of JS. We use JSX, an extension of vanilla JS, to write our components.

Why React?

Using components allow us to separate functionality and code.

It has a virtual DOM, to render faster.

Include the compiler Babel and Webpack, the module bundler. Babel compiles JSX down to React.createElement() calls.

Create a new react app and run npm install and npm start.

Components

Conceptually, components are like JavaScript functions. They accept arbitrary inputs (called ‚Äúprops‚ÄĚ) and return React elements describing what should appear on the screen. –reactjs.org

*React elements are the objects created by React.createElement()

Types of components

  • If props and state are not changing and we don’t need special work updating our component we can use PureCompnent.
  • If we don’t need to use state and lifecycle methods we can use a functional component. In order to use state with have to inherit from Component, and functional components don’t do that.

Functional components don’t have state but have props, we have to explicitly pass them as the argument for the function.

We can use object destructuring and extract values from props.

The components are the files written using JSX. You can import one file/component into another component an make it its parent component so its parent can render it inside an element.

Components:

*Always start component names with a capital letter.

*must never modify its own props. For that, use the state.

All React components must act like pure functions with respect to their props. –reactjs

*I would start writing the component as a function component and if class component features, like .state, are necessary, then change it to a class component.

Container and presentation components

So we have talked so far about functional, class and pure components. Container and presentational components are not different types of components, they are a way to organise your react app.

Presentational components usually receive props and display content.

Containers keep more complex logic, dealing with state and usually are class components.

Render a component

In this example I render the App component.

Importing from the node_modules folder and the src folder, exporting and the Component Chain

When a component doesn’t have an export means that is the top of this chain.

*For an optimization of udpating, React recommends using React.PureComponent instead of React.Component. PureComponent doesn’t have access to shouldComponentUpdate¬†because he will check for changes in props and state and update if necessary.

Props and state

Props are arguments passed into React components. They are like function arguments in JS and attributes in HTML. To send the props into a component, we use the same syntax as HTML attributes. React props are read-only. A component can change its own state but not its props. The parent who is passing the props can change them.

Default values for props:

State is similar to props, but it is private and fully controlled by the component. React components has a built-in state object. The state object is where you store property values that belongs to the component. When the state object changes, the component re-renders.

The state object is initialised in the constructor method.

We should call super() in the constructor since we are inheriting from another class via the extends keyword.  We set initial state in the constructor because it runs first.

We can change the state using setState(), which will re-render the component:

When we want to update our state based in the previous state. Never use a this.state inside a setState. We can pass a function to setState that, when called inside setState will be passed the component state from when that setState was called (previous state) and the return value will be the new state (important use return!).

React’s Event System

React uses basic HTML events and wrap them in the SyntheticEvent to make sure events sr handled the same way across all browsers.

setState() sets state asynchronously.

Update a nested state

We can not modify props only the state. Every component can modify its own state using setState, but what if a child want to modify the state of its parent? In order to do this we can use a callback. The parent component can pass down a callback function to its child where its state is modified. The child will have access to the cb inside the props being passed to it.

Event pooling

When the event fires, the event data is sent to the callback. If we need to use this data in an asynchronous way we can store it in a variable or we can make the event persistent (event.persist()).

Forms

When you have a form in React you usually want to write a JS function to handle the submission and have access to the form data. There are two different ways to control the form’s data, the controlled and the uncontrolled component,¬† but the safest is using a controlled form.

How to know if the component is controlled or uncontrolled? If the input has a value, it is a controlled component and if the input has a defaultValue is an uncontrolled component.

Controlled forms store their input values in the state. with a controlled component we can control user input and show errors. How do we add the input data to our state?

  1. Adding a value attribute to our input like <input type="text" name="query" onChange="{this.handleChange} value="{this.state.query}"/> or for radio and checkbox types checked attribute:
  2. and then updating our state with the setState method inside an onChange event listener that will fire every time the value of the input changes.

Having the form data in our state means that it is easy to pass them down as props to other component or pass them to a parent component using a function callback supplied in props.

Form with multiple inputs

We can access the event.target attributes of our handleChange function and get different info. For example if we give to our input an attribute name we can access it via event.target.nameand that means that we can create a more abstract handleChange function and use it without having to write a different function to handle the changes for each input of our form, in case we have a form with more than one input like the one in the example. In order to do so we have to make sure that the name value is equal to the key/keys of our state.

Lists

How to render a list of elements:

We should include a key (string) in our list of elements.

A good rule of thumb is that elements inside the map() call need keys. reactjs.org

React component methods

  • render()
  • Error handling during rendering, in a lifecycle method, constructor of any child component:
    • static getDerivedStateFromError()
    • componentDidCatch()
  • Other APIs:
    • setState()
    • forceUpdate()
  • Class properties:
    • defaultProps
    • displayName
  • Instance properties:
    • props
    • state

The render() method is necessary for a component to be valid, the other ones are optional.

React component lifecycle

Component lifecycle: creation(mounting), updating, and deletion(unmointing).

React has special built-in events relating to the component lifecycle, the lifecycle hooks/methods.

  • Pre-mounting:
    • constructor(). Where we create the initial state of the component.
  • Mounting:
    • static getDerivedStateFromProps(props, state). I gives us access to state and props.
    • render(). It returns JSX so React can insert it into the DOM.
    • componentDidMount(). Where we set up long-running or asynchronous processes such as fetching or updating data.
  • Updating:
    • static getDerivedStateFromProps(props, state). Where we can check state changes prior to an update. Use it when you want to modify the state based in the new props.
    • shouldComponentUpdate(nextProps, nextState). Invoked before the re-render. We can compare here if there are changes in the props or the state and avoid an unnecessary re-rendering.
    • render().Here the next props and the state became available as this.props and this.state. the component gets rendered into React’s virtual DOM.
    • getSnapshotBeforeUpdate(prevProps, prevState). Just before React commits content from its virtual DOM to the actual DOMReturns a snapshot that can be used in the next hook. You can get info like the scroll position.
    • componentDidUpdate(prevProps, prevState, snapshot). We can take some actions here without triggering a re-render of the component, like focusing on a specific form input. Here we have access to the previous and the current props and state and we can use this hook to update any third party libraries if they need to update due to the re-render.
  • Unmounting:
      • componentWillUnmount(). To clear any stuff we set up in componentDidMount.And avoid automatic data fetching for example that we don’t want.

     

Sending fetch requests

We can send a fetch request when the component is mounted, in the componentDidMount hook or we can tie it into an event.

Another way to organise the fetched data is using the container component pattern. Here it is a good link about it.

React Router

React router uses client-side routing, not server-side routing.

With Client-Side routing, it is now the responsibility of the Client-Side-Code to handle the routing, fetching and displaying of the data in the browser instead of the server.

Using pushState(state, title, url).

React and Babel

Babel is a compiler that makes all JS versions, even JSX, and emit a common one.

React and Webpack

Webpack combines different JS files, pre-bundles them together into a single file, to make less http requests to the server.

Reconciliation

React created during the first render the current tree,  that looks like the current DOM, and when updates cause a re-render it creates the workInProgress tree, representing how the DOM will look. React uses a diffing algorithm to check for differences and if necessary uses this second tree to update the current tree and the DOM .