2 - Reacting to changes


Last week we looked at how to write a HelloMentor React component (interactive example):

function Greeting() {
  return <span>Hello</span>;
function Mentor(props) {
  return <span>{props.name}</span>;
import Greeting from "./Greeting";
import Mentor from "./Mentor";

function HelloMentor() {
  return (
      <Greeting />
      <Mentor name="Ali" />

Handling events

So far we have only looked at React apps that are "static": they don't respond to user input. This week we will look at making our apps dynamic.

Recap: First-class functions in JavaScript

Before we look more at React we need to recap a concept in JavaScript. You may remember that functions in JavaScript are "first class" - that means we can pass a reference to a function (as a variable) and then call it elsewhere. Let's look at an example (interactive example):

function hello() {
  return "Hello!";

console.log(hello); // Logs: "ƒ hello() {}"
console.log(hello()); // Logs: "Hello!"

In the example above hello is a reference to a function. In the first console.log we log out the whole function. The function is not called until we use parentheses (), so we only log the string "Hello!" in the second console.log.

This is a really important and useful in React, as we can make a function and pass it to React so that it can call it when a user interacts with our app.

Event handlers in components

In previous lessons we learned how to attach event listeners with addEventListener:

// Create an event handler
function logWhenClicked() {
  console.log("buttonElement was clicked!");

// Listen for events and call the event handler when triggered
buttonElement.addEventListener("click", logWhenClicked);

We still need to listen to events in React, but event handlers are set up in a slightly different way (interactive example):

function ClickLogger() {
  function logWhenClicked() {
    console.log("Button was clicked!");

  return <button onClick={logWhenClicked}>Click me!</button>;

Every element in React has some special props that start with on that can be assigned to a function which will be called when the event is triggered.

Here's a few examples (a full list is available here):

  • onClick - the element was clicked

  • onCopy - the clipboard is used to copy some text

  • onKeyDown - a key is pressed down

  • onBlur - the element loses "focus"

  • onChange - only available for <input> & <select> (and a few others), triggered when changed

  • onDoubleClick - the element was double-clicked!

  • onPlay - a video starts playing

  • onSubmit - a form element is submitted

Notice that just like with addEventListener above, we pass the function reference to onClick instead of calling the function. If we call the function, it will run the function when we render, not when the user clicks on the button. (Remember that rendering is the term in React for inserting into the DOM).

:::tip Think of it like this: we give the event handler to React, so that React can call our function when the element is clicked. :::

Exercise A (estimate: 10 min)

1. Open the pokedex React application from last week and open the Logo.js file.

2. Add a function named logWhenClicked within the Logo component. (Hint: look at the example above).

3. In the logWhenClicked function, console.log a message (it doesn't matter what the message is).

4. Add an onClick handler to the <img> that will call logWhenClicked. (Hint: look at the ClickLogger component above).

5. In your web browser, try clicking on the logo image. What do you see in the JavaScript console?

6. In a group of 2 - 3 students, discuss what would happen if you changed your code to onClick={logWhenClicked()}. Can you explain why?

7. Report your discussion back to the rest of the class.

Passing Functions as Props

Sometimes we need to pass a function to another component as a prop, so that it can handle the event.

A common example for this is a Button component. This component adds some styling to a normal <button>, but still needs to be able to pass an event handler function to onClick. Let's look at an example (interactive example):

function ClickLoggerApp() {
  function logWhenClicked() {
    console.log("Button was clicked");

  return (
      <FancyButton handleClick={logWhenClicked} />
      <p>Then look in the console.</p>

function FancyButton(props) {
  return (
    <button className="my-fancy-classname" onClick={props.handleClick}>
      Click Me!

Notice how this is very similar to the example above where we created the handler and used it in the same component? The only difference here is that we are passing the function reference through a prop. We could even pass it through multiple components as props.

Exercise B (estimate: 10 min)

1. Open the pokedex React application and open the Logo.js file.

2. Copy and paste the logWhenClicked function from the Logo component to the App component.

3. Pass the logWhenClicked function reference as a prop to the Logo component. (Hint: look at the ClickLoggerApp component above for an example).

4. In the Logo component change the onClick prop so that it passes props.handleClick. (Hint: look at the FancyButton component above for an example).

5. In a group of 2 - 3 students, discuss what you think will happen when you click the logo image now. Can you explain why?

6. Report back to the rest of the class what you thought was going to happen and why.

Re-rendering components

So far we've seen that when the page loads, React calls our function components. The JSX elements that are returned from the component functions are turned into the DOM for you by React.

To be able to react to changes, we need to re-render our function components to get different JSX elements. React can then update the DOM based on the new JSX elements.

Let's look at how a component is re-rendered (interactive version):

function Counter(props) {
  console.log(`Rendering. props.likeCount is ${props.likeCount}`);

  return <button id="like-button">Likes: {props.likeCount}</button>;

If you look in the console, you'll see that the component is rendered once when the page loads. props.likeCount starts at 0, so React inserts "Count: 0" into the DOM.

We won't look at how this works at the moment, but behind the scenes there is some code that will listen for clicks on the button and force React to update. That means when you click the button, the function component is called again (or re-rendered).

Now props.likeCount is 1. React now updates the DOM to make sure it shows the correct number. Every time we click the button, the function component is called and React updates the DOM for us.

We don't need to worry about changing the DOM ourselves! This is what makes React so powerful. Even better, React will figure out exactly the right bits of the DOM that need to be changed, a concept called the "virtual DOM". This makes it extremely efficient and fast.


State is a general concept in software engineering. It is used when a part of your app needs to "remember" something that changes when people interact with it.

This is a simple example, but if we had lots of bits of state, then we can make very complex apps.

React Hooks

React has built-in functionality for initializing and updating state in our components. We will access state via a React Hook called useState.

Hooks are a newish feature in React. You may find older tutorials that don't use Hooks, but don't panic. The concepts we learn here are the same whether or not you use Hooks. We are looking at Hooks first because they are simpler to learn for beginners.

Importing useState

To be able to access the useState Hook, we first need to import it from the React package. Let's look at an example (interactive example):

import React, { useState } from "react";


If we look at the console, useState is just a function. It lives inside the React code that you installed when you created the app.

To reference the useState function in our component, we need to import it from the React code. The curly braces around useState are a bit like writing:

import React from "react";
let useState = React.useState;

In fact we can just write React.useState in our component if we want! But to type a bit less code, we import it (using the curly braces) once and then can just use useState.

Using useState

Now let's look at how we can use the useState Hook (interactive example):

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

  return <p>You clicked {count} times</p>;

Let's break this down into small pieces. First, let's look at calling the useState function:


This initializes the state variable to 0. Any parameter passed to useState will be used as the initial value.

Next, let's look at how we render the state variable in our component:

return <p>You clicked {count} times</p>;

count is just a variable, so to insert it into our JSX we treat it like any other variable: we use curly braces.

Finally, let's look at how we get hold of the count variable:

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

To fully understand this bit of code, we first have to understand destructuring. Let's look at this blog post by Wes Bos about array destructuring.

Now we can understand that useState is returning an array, with two items. The first item in the array is the current value of the count state. In our example it will be 0 on the first render. The second item in the array is a function that we will use to update our state.

Tip: Follow the useState naming convention.

When we destructure an array, we can name the variables whatever we want, but there is a naming convention when destructuring the useState array. The first variable should be named whatever your state is called, and the second variable should be the same name but prefixed with set. Let's look at some examples:

const [userIsLoggedIn, setUserIsLoggedIn] = useState(false);

const [username, setUsername] = useState("chris");

const [unreadMessages, setUnreadMessages] = useState(5);
Exercise C (estimate: 5 min)

2. Take a few minutes to read the code. Why do you think the app is broken?

3. Initialize a new state variable with useState that will fix the app. Think carefully about how you should name the variables.

4. Discuss with another student how you would create another state variable that represents the weather conditions (e.g. sunny, rain).

Updating State

Our Counter isn't very useful right now! Let's make it more useful by getting count to actually count up (interactive example):

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

  function incrementCount() {
    setCount(count + 1);

  return (
      <button onClick={incrementCount}>Click me</button>
      <p>You clicked {count} times</p>

Our component now has a <button>, which will call the incrementCount function when clicked:

<button onClick={incrementCount}>Click me</button>

The incrementCount function then calculates the new state by adding 1 onto the current count. And then calls setCount to set the new state:

function incrementCount() {
  setCount(count + 1);

setCount does two things. First, it updates the state that our component is "remembering". Whatever you pass as the argument to setCount will be remembered as the new state.

It also tells React that the old state that is still shown in the DOM is outdated and so the DOM needs to change. Because of this, React will re-render all of our components to figure out what to change in the DOM.

When re-rendering, React will call our Counter component again, but this time when we call useState it will give us the updated state of 1, instead of the initial state of 0:

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


On the second render, count is now set to 1. Every time we click the button, the whole cycle starts again.

Exercise D (estimate: 15 min)

1. Open the pokedex React application and open the CaughtPokemon.js file.

2. Create a new state variable with useState. It should be named caught and be initialized to 0

3. Within the JSX, there should be a "hard-coded" number 0. Replace it with your new caught state.

4. Add a button to the component with an onClick handler that calls a function called catchPokemon.

5. Create the catchPokemon function and have it update the caught state so that it is increased by 1 on each click. HINT You will need to call the set state function (the 2nd item in the useState array) with caught + 1.

6. Write down the things that will happen when you click the button. Compare your list with another student and discuss. HINT The state will be updated to be the current state + 1. React is notified that our state has changed, so it re-renders. When rendering, the current state will be different and so React updates the DOM.

Don't mutate State

As we just learned, setCount updates the state for us, but it also notifies React of changes. If you try to just change the count variable without using setState, nothing will happen, because React wouldn't be notified of the change. You can only modify (or mutate) state using the setter function (interactive example):

function Counter() {
  let [count, setCount] = useState(0);

  function handleClick() {
    count = count + 1;

  return (
      Count: {count}
      <button onClick={handleClick}>Click</button>

Where does State live?

We have talked about how a component "remembers" state. In fact, each component instance remembers separate state from other components. This means we can have multiple different Counters, each with a different state (interactive example):

function App() {
  return (
      <Counter />
      <Counter />
      <Counter />

More complex states

The examples we've looked at so far have used numbers, strings and Booleans. You can also use arrays and objects in state too. Let's take a look at an example (interactive example).

function ShoppingList() {
  const [list, setList] = useState([]);

  function addToList() {

  return (
      <button onClick={addToList}>Add item to list</button>
      {list.map((item, index) => {
        return <li key={index}>{item}</li>;

In this shopping list example, we're initializing the list state to be an empty array. To display our list we loop through the array (like we learned last week) and render an <li> for each item in the list.

When we want to add something to the list, we can use the list.concat method to make a new array with the new item. This new array is then set as the new state. Right now, our example is not very useful as it can only add Bread to the list! Next week, we'll look at how we can allow users to write their own items to the list.

Tip : Don't use the array push method with state. Instead use the array concat method.

The list.push method won't work here, as this method mutates the existing array. React requires a completely new array to be set as the new state, otherwise it doesn't realize that the value has changed. The concat method works because it copies the whole existing array to a brand new array before it adds the new item.

Exercise E (estimate: 10 min)

1. Open the pokedex React application and open the CaughtPokemon.js file.

2. Change the useState to be initialized to an empty array ([])

3. There will now be a bug in your app! We don't see how many Pokémon we have caught. Discuss with another student what you think the problem is.

4. Change the JSX to instead render caught.length. Does this fix the bug?

5. Let's now show the names of the Pokémon we have caught. Render a <ul> element within the component. Then use the map method to loop through each item in the caught array and render it in an <li> element.

6. Change the catchPokemon function to add a new Pokémon (it doesn't matter which one) onto the caught array. (Hint: use the concat method.)

7. (STRETCH GOAL) Generate a random Pokémon each time you click the button. Check this post if you're stuck StackOverflow post.

Setting multiple States

So far we've only seen an example with one state variable. But you can create multiple state variables if you want! Let's see an example (interactive example):

function Weather() {
  const [temperature, setTemperature] = useState(15);
  const [conditions, setConditions] = useState("sunny");

  function makeItHotter() {
    setTemperature(temperature + 1);
  function makeItColder() {
    setTemperature(temperature - 1);

  function makeItSunny() {
  function makeItRainy() {

  return (
      <h1>Today's Weather</h1>
        <li>Temperature will be {temperature}°C</li>
        <li>The conditions will be {conditions}</li>

      <hr />
      <button onClick={makeItHotter}>+</button>
      <button onClick={makeItColder}>-</button>
      <button onClick={makeItSunny}>Make it sunny</button>
      <button onClick={makeItRainy}>Make it rainy</button>

When do you use Props or State?

We've looked at the 2 main ways of managing data in our React components. But when should we use props and when should we use state?

Remember that props are like "arguments" to a component. It's good practice to make sure that you don't modify arguments after you receive them. Just like state, React prevents you from mutating them. Let's have a look at an example (interactive example):

function Greeting(props) {
  function handleThing() {
    props.name = "Mozart";

  return (
      <p>Hello {props.name}</p>
      <button onClick={handleThing}>Click me</button>

When you click the button, you might expect the name prop to change to "Mozart". But it doesn't! React has made props read-only, which is a reminder that we shouldn't change props.

If we were allowed to change props, React doesn't have a way of telling that we've changed the data. Our UI is now stale - not up-to-date with the latest data - and has no way of knowing that it has to re-render.

From this we can get a clue about when to use state. If data changes over time, then we need to use state. My rule of thumb is that I always use props until I know that it needs to change over time, then I convert it to state.

Exercise F (estimate: 5 min)

2. Take a few minutes to read the code. Discuss with another student what you think will happen when you click on the highlighted word.

3. Now click on the highlighted word. Can you explain why this happened?

4. Fix the app so that clicking on the highlighted word will change the highlight color to purple.

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