Intro to State

Learning Objectives

After this lesson you will be able to:

Explain what state is and how to implement it in React
Use Array Destructuring to create variables from an array
Update state and re-render the Component

Framing

The best analogy to understand React state is to start by answering the following question: How are you feeling this very moment?

Are you happy to be in class learning a new topic?
Are you frustrated having to sit in class for the next several hours learning even more React?
Did some random person say hello out of the blue make you smile?

The answer to any one of those questions has a direct impact on your state of mind. A happy state will be reflected in your smile, tone of voice, being nice to others in return. An unhappy state will have the opposite effect.

As with all human beings our state of mind can change on the fly which is almost always reflected in our facial expressions or actions. Applications also have a state which is reflected in the UI presented to the user.

Therefore updating an applications state is our control mechanism for how we update the UI.

Props Recap

So far we've worked with props and used them to pass values from a parent to a child Component. This pattern of passing data down will be consistent in React as the flow of data is unidirectional and always flows down.

We also know that the props passed down to a child are organized, by React, into an object where every prop becomes a key: value pair.

Props are a great way to pass data but have the following limitations:

Data in React is unidirectional and props are always passed down from parent > child
Props are immutable
Reassigning a prop value will have no effect on the Component.
Child Components cannot communicate directly and therefore cannot pass data between them

🔎 Check for Understanding

Take a a minute to think about the following questions:
- What do we use props for?
- What limitations do props have?
- Is there any best practice you can think of when creating a prop?

When asked slack your answer(s) in a thread created by the instructor

Intro To State

In our attempt to provide a coherent framing of React state the point was made that what you see on the page is the current version of the applications state. Any changes to state will then be reflected in the UI.

One important thing to note here is that any changes to state will cause the Component to re-render.

This is essentially how the UI is updated.

This is a very important concept to keep in mind as a re-render can also initiate additional function calls, something we will discuss as part of Reacts lifecycle methods.

Rules Of State

🚔 Here are the rules we need to follow when working with state.

State is assigned using the useState or useReducer hook
The State value can be assigned any data type
The State value is never updated directly but only using its corresponding setState function
The state value must always be overwritten with a new value

Working With State

So updating state will, most often, require the user to interact with the application. Hence, the user performs some action, like clicking a button, and the component responds by doing a thing and then updating state.

Now it's time to dive in and build a small Counter app that contains state.

A Simple Counter Component

We'll walk through building a very simple Counter Component which will do the following:

Provide the user 2 buttons to increment or decrement
Display the initial and update value as it changes

Spin Up A New CodeSandbox

For this demo you will spin up a new CodeSandbox.

To do this just click on the blue Create Sandbox button on the right of the page.

Now scroll down to the Official Templates section and choose React by CodeSandbox.

Creating The Counter Component

⏰ Activity - 3min

Since you already have experience creating Components take a minute to perform the following activity.

Counter Component

Create a new file in src called Counter.js
Import React
Create the Component
Return the following JSX:

<>
    <span>Current Count: 0</span>
    <section>
        <button>+</button>
        <button>-</button>
    </section>
</>

Export the Component

App Component

Import the Component into App.js
Replace all the JSX inside of className="App" with the Counter Component.

<div className='App'>
	<Counter />
</div>

Once you're done, React should render the following:

That JSX looks like it could use a little styling.

So let's copy/paste the following css to styles.css:

CSS

.App {
	font-family: sans-serif;
	text-align: center;
	width: 160px;
	margin: auto;
	display: flex;
	flex-direction: column;
}

section {
	display: flex;
}

button {
	flex: 1;
}

span {
	font-size: 20px;
}

And now the design should update to look like:

The `useState` Hook

In order to add state to the Counter Component we will first need to import the useState Hook from React. useState is one of the 3 Basic Hooks as per the Official React Doc.

A Word On Hooks

Hooks were introduced in React Version 16.8. Before hooks, all state needed to be placed within a Class component.

Hooks introduce state management to Functional Components, using a simpler and more flexible API and let you split one component into smaller functions based on what pieces were needed.

⏰ Activity - 2min

Since we will be working with Hooks solely in this class let's take a minute to review the following React Docs:

Hooks API Reference - all the available Hooks.

Class Component State Example

Class Components come with a lot of boilerplate, which can feel bulky, especially when dealing with a simpler state.

Instructor Demo

The instructor will perform a small demo of creating a class based Component that includes state. Not need to code along as none of the following code will be used in building the Counter App.

Here is how state would have been configured using a class Component.

class Counter extends React.Component {
  constructor(props) {
    super(props)
    this.state = { count: 0 }
  }
  render() {}
}

Although with the the next most recent ES (ECMA Script) class properties proposal and the help ofBabel to transpile the code the above could be written as:

class Counter extends React.Component {
  state = { count: 0 }
  render() {}
}

State of Transition

We are currently in a state of transition in world of React.

Hooks were a game changer and have become the tool of choice when writing React in 2020.

Keep in mind however that there is way more code out there written in the previous syntax but Hooks and any research you perform on React will almost certainly show Class based solutions unless you include the keyword Hook in your search query.

Instructor Demo

The instructor will perform a small demo of performing a Google search for updating state in React, with and without the keyword Hook

Importing `useState`

Now it's time to import useState into the Counter Component.

The React library that is imported in every Component has a key called useState that we elicit from React and store in a variable of the same name.

If we take a moment to console log React in the Counter Component:

import React from 'react';
console.log('this is React', React);

We would see everything it has to offer include useState

Since we are familiar with Object Destructuring let's use it to elicit the value of this key and store in a variable simultaneously.

import React, { useState } from 'react';

Just so that we get a better idea of what useState actually is let's add a console log.

const Counter = () => {
  console.log('useState - ', useState);
  // ...rest of code
}

The output should look like the following:

It appears that useState is a function that takes in in initialState, calls a supporting function and returns dispatcher.useState().

We won't get into the underlying code here but one thing to to highlight is the keyword dispatcher.

We will revisit this concept later when we cover the useReducer hook as it uses a similar naming convention of dispatch for corresponding setState function.

`useState` Rules and Best Practices

Let's take a moment to once again review the rules of useState and include some best practices as well.

🚔 Rules

the State value is never updated directly
the State value is only updated using it's corresponding setState function
the state value must always be overwritten with a new value

⭐️ Best Practices

Use Array Destructuring when initializing the state variables
Name the initial state based on what it contains
Use the same name for the function but precede it with the word set
Use a the callback function version of useState if you need to reference the previous version of state
Give thought as to what needs to be in state and how that state should be organized and structured
Always use ...spread operator to copy object and array values to the new state

Creating An Instance Of State

With useState imported it's time to create an instance of state. To do this we will call useState() and pass it an initial starting value of 0.

⭐️ Name the initial state based on what it contains.

const countState = useState(0);

Once again let's add a console log and see what it returns.

const countState = useState(0);
console.log('countState -', countState);

We should see the following:

So, it appears countState is set to an array that contains the following elements:

0 - The initial state value we defined
1 - function - which will be used to update state.

One way to create 2 new variables based on the array would be to manually elicit their values using standard array bracket notation.

In keeping with best practices we will name the initial state variable count as it will be used it to increment/decrement a starting value essentially keep count.

⭐️ Use the same name for the function but precede it with the word set

Of course, the corresponding function that will be used to update state should be called setCount.

⭐️ Use the same name for the function but precede it with set

const count = countState[0];
const setCount = countState[1];

Array Destructuring

⭐️ A more convenient way of doing this is using ES6 Array Destructuring.

Array Destructuring elicits the values from the array based on their position and stores them in variables.

const [count, setCount] = useState(0);

Using State

Now that our initial value is been assigned to the count variable let's update the JSX to use that value instead of a current hard coded value of 0.

Of course, as has been stated several times already, JSX requires that all JavaScript be surrounded in curly braces.

return (
  <div>
    <span>Current Count: {count}</span>
    ... rest of code
  </div>
);

Updating State

With our state value in place it's time to provide some functionality to the buttons and allow the user a means to interact with the app and update state.

In the case of our Counter the only way to update count is to call the setCount function and pass it a new value.

🚔 Always use the setState function to update state

There are 2 ways to perform this action:

// grab the current version of state
setCount(count + 1);

// OR

// use a callback function and pass the previous version of state
setCount((prevState) => prevState + 1);

In the second example the setter function takes in an callback function that is passed the previous value of state and returns a new value altogether.

The argument in this example is called prevState by convention but you can name it anything you want.

There are scenarios when the callback function version is required. One example would be when state is being updated within the callbacks of either a setTimeout() or setInterval().

Since that isn't the case here we will use the first example to update state.

Adding an `onClick` Event

In order to allow the user to interact with the buttons we will need to add an event listener.

React event listeners are an additional topic we will revisit again in future lessons. They are essentially synthetic events based on the real underlying JS events and perform the same operations as before. Events you might have worked with before are:

click => onClick
submit => onSubmit
change => onChange
mouseover => onMouseOver

For now we will add an onClick event listener that calls setCount to update state.

Also, as with plain JavaScript or jQuery we will use an anonymous callback to pause the execution until the click event has occurred.

  return (
    <div>
      <span>Current Count: {count}</span>
      <section>
        <button onClick={() => setCount(count + 1)}>+</button>
        <button onClick={() => setCount(count - 1)}>-</button>
      </section>
    </div>
  );

If we test out the app we should see that the count value will change based on user input.

Instructor Demo

The instructor will demo where React is caught in an infinite loop that was triggered by updating state without using the onClick callback funtion

Event Handlers

In order for us to use the React library effectively there are certain conventions we must practice.

This is a good thing in that we can quickly examine code and expect some consistency in how it is written.

However, some React code is written solely based on the adoption of the community at large.

One example of a particular convention we use is when creating event handler functions.

The convention is to precede their name with word handle.

Let's give that a try by creating the following supporting functions:

handleIncrement
handleDecrement

const handleIncrement = () => {};

const handleDecrement = () => {};

Conclusion

Before we wrap up this lesson, let's move the setState function calls into their corresponding handler functions and update the onClick to reflect this refactor.

const handleIncrement = () => {
	setCount(count + 1);
};

const handleDecrement = () => {
	setCount(count - 1);
};

Intro to State

Learning Objectives

Framing

Props Recap

🔎 Check for Understanding

Intro To State

Rules Of State

Working With State

A Simple Counter Component

Spin Up A New CodeSandbox

Creating The Counter Component

⏰ Activity - 3min

The useState Hook

A Word On Hooks

⏰ Activity - 2min

Importing useState

useState Rules and Best Practices

Creating An Instance Of State

Array Destructuring

Using State

Updating State

Adding an onClick Event

Event Handlers

Conclusion

The `useState` Hook

Importing `useState`

`useState` Rules and Best Practices

Adding an `onClick` Event