Componentizing our React app

Defining a component can seem tricky until you have some practice, but the gist is:

• If it represents an obvious "chunk" of your app, it's probably a component
• If it gets reused often, it's probably a component.

That second bullet is especially valuable: making a component out of common UI elements allows you to change your code in one place and see those changes everywhere that component is used. You don't have to break everything out into components right away, either. Let's take the second bullet point as inspiration and make a component out of the most reused, most important piece of the UI: a todo list item.

Before we can make a component, we should create a new file for it. In fact, we should make a directory just for our components. Make sure you're in the root of your app before you run these commands!

# create a `components` directory
mkdir src/components
# within `components`, create a file called `Todo.jsx`
touch src/components/Todo.jsx

Don't forget to restart your development server if you stopped it to run the previous commands!

Let's add a Todo() function in Todo.jsx. Here, we define a function and export it:

function Todo() {}

export default Todo;

This is OK so far, but our component should return something useful! Go back to src/App.jsx, copy the first <li> from inside the unordered list, and paste it into Todo.jsx so that it reads like this:

function Todo() {
  return (
    <li className="todo stack-small">
      <div className="c-cb">
        <input id="todo-0" type="checkbox" defaultChecked />
        <label className="todo-label" htmlFor="todo-0">
          Eat
        </label>
      </div>
      <div className="btn-group">
        <button type="button" className="btn">
          Edit <span className="visually-hidden">Eat</span>
        </button>
        <button type="button" className="btn btn__danger">
          Delete <span className="visually-hidden">Eat</span>
        </button>
      </div>
    </li>
  );
}

export default Todo;

Now we have something we can use. In App.jsx, add the following line at the top of the file to import Todo:

import Todo from "./components/Todo";

With this component imported, you can replace all of the <li> elements in App.jsx with <Todo /> component calls. Your <ul> should read like this:

<ul
  role="list"
  className="todo-list stack-large stack-exception"
  aria-labelledby="list-heading">
  <Todo />
  <Todo />
  <Todo />
</ul>

When you return to your app, you'll notice something unfortunate: your list now repeats the first task three times!

We don't only want to eat; we have other things to — well — to do. Next we'll look at how we can make different component calls render unique content.

Components are powerful because they let us re-use pieces of our UI, and refer to one place for the source of that UI. The problem is, we don't typically want to reuse all of each component; we want to reuse most parts, and change small pieces. This is where props come in.

In order to track the names of tasks we want to complete, we should ensure that each <Todo /> component renders a unique name.

In App.jsx, give each <Todo /> a name prop. Let's use the names of our tasks that we had before:

<Todo name="Eat" />
<Todo name="Sleep" />
<Todo name="Repeat" />

When your browser refreshes, you will see… the exact same thing as before. We gave our <Todo /> some props, but we aren't using them yet. Let's go back to Todo.jsx and remedy that.

First modify your Todo() function definition so that it takes props as a parameter. You can console.log() your props if you'd like to check that they are being received by the component correctly.

Once you're confident that your component is getting its props, you can replace every occurrence of Eat with your name prop by reading props.name. Remember: props.name is a JSX expression, so you'll need to wrap it in curly braces.

Putting all that together, your Todo() function should read like this:

function Todo(props) {
  return (
    <li className="todo stack-small">
      <div className="c-cb">
        <input id="todo-0" type="checkbox" defaultChecked={true} />
        <label className="todo-label" htmlFor="todo-0">
          {props.name}
        </label>
      </div>
      <div className="btn-group">
        <button type="button" className="btn">
          Edit <span className="visually-hidden">{props.name}</span>
        </button>
        <button type="button" className="btn btn__danger">
          Delete <span className="visually-hidden">{props.name}</span>
        </button>
      </div>
    </li>
  );
}

export default Todo;

Now your browser should show three unique tasks. Another problem remains though: they're all still checked by default.

In our original static list, only Eat was checked. Once again, we want to reuse most of the UI that makes up a <Todo /> component, but change one thing. That's a good job for another prop! Give your first <Todo /> call a boolean prop of completed, and leave the other two as they are.

<Todo name="Eat" completed />
<Todo name="Sleep" />
<Todo name="Repeat" />

As before, we must go back to Todo.jsx to actually use these props. Change the defaultChecked attribute on the <input /> so that its value is equal to the completed prop. Once you're done, the Todo component's <input /> element will read like this:

<input id="todo-0" type="checkbox" defaultChecked={props.completed} />

And your browser should update to show only Eat being checked:

If you change each <Todo /> component's completed prop, your browser will check or uncheck the equivalent rendered checkboxes accordingly.

We have still another problem: our <Todo /> component gives every task an id attribute of todo-0. This is bad for a couple of reasons:

• id attributes must be unique (they are used as unique identifiers for document fragments, by CSS, JavaScript, etc.).
• When ids are not unique, the functionality of label elements can break.

The second problem is affecting our app right now. If you click on the word "Sleep" next to the second checkbox, you'll notice the "Eat" checkbox toggles instead of the "Sleep" checkbox. This is because every checkbox's <label> element has an htmlFor attribute of todo-0. The <label>s only acknowledge the first element with a given id attribute, which causes the problem you see when clicking on the other labels.

We had unique id attributes before we created the <Todo /> component. Let's bring them back, following the format of todo-i, where i gets larger by one every time. Update the Todo component instances inside App.jsx to add in id props, as follows:

<Todo name="Eat" id="todo-0" completed />
<Todo name="Sleep" id="todo-1" />
<Todo name="Repeat" id="todo-2" />

Now go back to Todo.jsx and make use of the id prop. It needs to replace the <input /> element's id attribute value, as well as its <label>'s htmlFor attribute value:

<div className="c-cb">
  <input id={props.id} type="checkbox" defaultChecked={props.completed} />
  <label className="todo-label" htmlFor={props.id}>
    {props.name}
  </label>
</div>

With these fixes in place, clicking on the labels next to each checkbox will do what we expect – check and uncheck the checkboxes next to those labels.

We're making good use of React so far, but we could do better! Our code is repetitive. The three lines that render our <Todo /> component are almost identical, with only one difference: the value of each prop.

We can clean up our code with one of JavaScript's core abilities: iteration. To use iteration, we should first re-think our tasks.

Each of our tasks currently contains three pieces of information: its name, whether it has been checked, and its unique ID. This data translates nicely to an object. Since we have more than one task, an array of objects would work well in representing this data.

In src/main.jsx, declare a new const beneath the final import, but above ReactDOM.createRoot():

const DATA = [
  { id: "todo-0", name: "Eat", completed: true },
  { id: "todo-1", name: "Sleep", completed: false },
  { id: "todo-2", name: "Repeat", completed: false },
];

Next, we'll pass DATA to <App /> as a prop, called tasks. Update your <App /> component call inside src/main.jsx to read like this:

<App tasks={DATA} />

The DATA array is now available inside the App component as props.tasks. You can console.log() it to check, if you'd like.

To render our array of objects, we have to turn each object into a <Todo /> component. JavaScript gives us an array method for transforming items into something else: Array.prototype.map().

Inside App.jsx, create a new const above the App() function's return statement called taskList. Let's start by transforming each task in the props.tasks array into its name. The ?. operator lets us perform optional chaining to check if props.tasks is undefined or null before attempting to create a new array of task names:

const taskList = props.tasks?.map((task) => task.name);

Let's try replacing all the children of the <ul> with taskList:

<ul
  role="list"
  className="todo-list stack-large stack-exception"
  aria-labelledby="list-heading">
  {taskList}
</ul>

This gets us some of the way towards showing all the components again, but we've got more work to do: the browser currently renders each task's name as plain text. We're missing our HTML structure — the <li> and its checkboxes and buttons!

To fix this, we need to return a <Todo /> component from our map() function — remember that JSX is JavaScript, so we can use it alongside any other, more familiar JavaScript syntax. Let's try the following instead of what we have already:

const taskList = props.tasks?.map((task) => <Todo />);

Look again at your app; now our tasks look more like they used to, but they're missing the names of the tasks themselves. Remember that each task we map over contains the id, name, and completed properties we want to pass into our <Todo /> component. If we put that knowledge together, we get code like this:

const taskList = props.tasks?.map((task) => (
  <Todo id={task.id} name={task.name} completed={task.completed} />
));

Now the app looks like it did before, and our code is less repetitive.

Now that React is rendering our tasks out of an array, it has to keep track of which one is which in order to render them properly. React tries to do its own guesswork to keep track of things, but we can help it out by passing a key prop to our <Todo /> components. key is a special prop that's managed by React – you cannot use the word key for any other purpose.

Because keys should be unique, we're going to re-use the id of each task object as its key. Update your taskList constant like so:

const taskList = props.tasks?.map((task) => (
  <Todo
    id={task.id}
    name={task.name}
    completed={task.completed}
    key={task.id}
  />
));

You should always pass a unique key to anything you render with iteration. Nothing obvious will change in your browser, but if you do not use unique keys, React will log warnings to your console and your app may behave strangely!

Now that we've got our most important component sorted out, we can turn the rest of our app into components. Remembering that components are either obvious pieces of UI, reused pieces of UI, or both, we can make two more components:

• <Form />
• <FilterButton />

Since we know we need both, we can batch some of the file creation work together in one terminal command. Run this command in your terminal, taking care that you're in the root directory of your app:

touch src/components/{Form,FilterButton}.jsx

Open components/Form.jsx and do the following:

• Declare a Form() function and export it at the end of the file.
• Copy the <form> tags and everything between them from inside App.jsx, and paste them inside Form()'s return statement.

Your Form.jsx file should read like this:

function Form() {
  return (
    <form>
      <h2 className="label-wrapper">
        <label htmlFor="new-todo-input" className="label__lg">
          What needs to be done?
        </label>
      </h2>
      <input
        type="text"
        id="new-todo-input"
        className="input input__lg"
        name="text"
        autoComplete="off"
      />
      <button type="submit" className="btn btn__primary btn__lg">
        Add
      </button>
    </form>
  );
}

export default Form;

Do the same things you did to create Form.jsx inside FilterButton.jsx, but call the component FilterButton() and copy the HTML for the first button inside <div className="filters btn-group stack-exception"> from App.jsx into the return statement.

The file should read like this:

function FilterButton() {
  return (
    <button type="button" className="btn toggle-btn" aria-pressed="true">
      <span className="visually-hidden">Show </span>
      <span>all </span>
      <span className="visually-hidden"> tasks</span>
    </button>
  );
}

export default FilterButton;

Let's make use of our new components. Add some more import statements to the top of App.jsx and reference the components we've just made. Then, update the return statement of App() so that it renders our components.

When you're done, App.jsx will read like this:

import Form from "./components/Form";
import FilterButton from "./components/FilterButton";
import Todo from "./components/Todo";

function App(props) {
  const taskList = props.tasks?.map((task) => (
    <Todo
      id={task.id}
      name={task.name}
      completed={task.completed}
      key={task.id}
    />
  ));
  return (
    <div className="todoapp stack-large">
      <h1>TodoMatic</h1>
      <Form />
      <div className="filters btn-group stack-exception">
        <FilterButton />
        <FilterButton />
        <FilterButton />
      </div>
      <h2 id="list-heading">3 tasks remaining</h2>
      <ul
        role="list"
        className="todo-list stack-large stack-exception"
        aria-labelledby="list-heading">
        {taskList}
      </ul>
    </div>
  );
}

export default App;

With this in place, your React app should render basically the same as it did before, but using your shiny new components.

And that's it for this article — we've gone into depth on how to break up your app nicely into components and render them efficiently. Next we'll look at handling events in React, and start adding some interactivity.

• Previous
• Overview: Understanding client-side JavaScript frameworks
• Next