Ethan Mick
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Completing your Todos

In this guide, you will add some functionality to your to-do app. You will mark tasks as completed! This feature will allow you to dive into using your app and see what new features you may want to add.

When thinking of a feature like this, it's essential to think through the requirements. For example, what are the user stories, and how should the feature function?

For checking an item off, the user story is:

As a user, I want to mark an item as complete.

You can detail the following requirements from that story:

  • Each list item should be complete or incomplete.
  • Each list item should display its status.
  • Each list item should have the functionality to mark it complete.

A big question is the addition of a second user story:

As a user, I want to mark an item as incomplete.

That story changes the functionality drastically. Without the last point, you could remove a list item from the list when completed. Poof! You don't need to track it anymore. However, you double the state you need to track with the second story. Can a user reorder completed items? Can they mark them as incomplete? Can they edit them?

These are great questions to ask, especially when working with your team or a client. Take the time to understand how the scope will change with what seems like a simple task.

So for this app? Of course, you'll do it! Exploding scope for everybody. That said, this section will only deal with completing an item. You'll mark it as incomplete in the following guide.

How to complete an item?

The next step is to consider how this will work. You need a way to mark an item as complete, and then later, incomplete. If you play with many To-Do apps, this is often either a gesture, swipe, or checkbox. To start, you'll do a simple check box.

Upon being checked, the item will disappear from the to-do list. Here are two ways to accomplish this:

  1. Have two arrays to manage the state of a list. Have one array for non-complete items and a second for complete list items.
  2. Have a property on the Todo object that tracks if the task has been completed or not. Depending on what the user is viewing, filter the list accordingly.

The first approach keeps the Todo object simple but complicates the state management at a higher level. For instance, a to-do list will need two internal lists to track all the items. This doubles the amount of work you will need to do to maintain your application! Be careful when these situations present themselves, and strive to keep your app simple.

The second option is a more straightforward approach and what you'll do here. It's also what you accounted for in the Components section when you made the Todo an object with properties instead of just a string.

First, add a new property to the Todo interface that will track if the item is done or not.

interface Todo {
text: string
done?: boolean

Making the property optional means you aren't forced to add it to every item. You won't need to set done explicitly; it's false when creating a new to-do. And since an undefined value is treated similarly to false in most instances, it's easy to infer the value for the missing key. Easier all around!

Next, we can update the TodoListItem to have a checkbox that, upon clicking, will trigger the item to be marked done. So go ahead and add a checkbox to the TodoListItem:

<input type="checkbox" />

In React, the input type will change the properties it requires to show the value. Since this is a checkbox, the input needs to know if it is checked or not, which is a boolean. When the input changes, the event it emits is also a boolean. The checked property is different from the input you are using for adding a new Todo, which takes and emits a string.

So instead of value, you will use checked to set if it is complete. You will use the same onChange handler to watch for the user clicking the checkbox.

When you use the done property on a Todo for checked, you still need an additional "hey, I changed" handler for when the user clicks the checkbox. This handler isn't a property of a Todo, so you need to augment the props for this component to add it in.

type TodoListItemProps = {
onChange: () => void
} & Todo

The above code creates a new type, TodoListItemProps, which combines the onChange handler and the rest of the properties in the Todo. This is a simple way of extending Types without literally using extends with interfaces. You can read more about unions and intersections here.

With these new props, the updated TodoListItem looks like this:

const TodoListItem = ({ text, done, onChange }: TodoListItemProps) => (
<input type="checkbox" checked={done} onChange={(e) => onChange()} />

Now that you've made these types required, you will need to update the parent to fix the build.

Checking off an item

The parent TodoList now needs to pass additional properties to the list item. The first, done, is handled for you in the Todo interface. But since you haven't added done: true to anything, all of the items are marked as not done. So you need to add the onChange handler to trigger the logic: "When a user clicks the checkbox, mark it as done and stop displaying it to the user."

Start by creating the handler in the TodoList component:

const onComplete = () => {}

The above code will change the state, so you need to use setTodos. Unfortunately, you can't just change the to-do value by doing todos[0].done = true. While that may look like it will work, you're changing the state without telling React about it. That will lead to bugs and unexpected behavior.

The challenge here is to know which Todo the user just completed. Do this in a few ways:

  1. The handler passes in Todo object itself and uses object equality to find the to-do in the array.
  2. The handler passes the array index and looks up the to-do by index.
  3. The handler passes in a unique identifier (ID) and looks up the to-do by ID.

The first is fragile because you rely on the object reference not to change. That might not be the case if you save things to local storage, make a network request, or update the internal state and return a new object. So, in general, this is a bad idea.

Using the array index is sometimes applicable, but you're going to run into issues here. Mutating an array and using the index as keys will cause trouble. This article covers why it is a bad idea to use indexes as keys.

So instead, you're going to take a different tack.

Add a unique ID

You should add a unique ID to each Todo that simulates the ID it would get from a database. It's a unique ID not tied to the index in the array, so React can better track the differences.

There are great packages you can install to get unique IDs, but that's a little overkill for what you need here. So instead of adding to your dependencies, you can use a small function that will generate a UUID with no external dependencies:

const UUID = () =>
'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, (c) => {
const r = (Math.random() * 16) | 0
const v = c == 'x' ? r : (r & 0x3) | 0x8
return v.toString(16)


This will generate a string like a60694f8-7e85-46c0-ad7c-7e14a8a22a6e. To use this, add a new id property to the Todo interface. Since it needs to be on each Todo, it can't be optional.

interface Todo {
id: string
text: string
done?: boolean

Then, when you create a new Todo, give it an ID:

const onAddTodo = (text: string) => {
setTodos([...todos, { text, id: uuid() }])

Great! Now you change up the map to use the ID instead of the index for the key:

{ => (
<TodoListItem key={} onChange={() => onComplete()} {...todo} />

Okay, but back to checking the list items off. Now you have a better way to mutate the array but no way to actually mutate it?

Except you do! You just added it. Since each ID is unique, you can look up the triggered Todo with the ID. Working backward, update the onComplete handler to account for this:

const onComplete = (id: string) => {
setTodos((todos) => {
const i = todos.findIndex((todo) => === id)
todos[i] = {
done: !todos[i].done,
return [...todos]

The above code uses the function version of a setter in React. When calling setState and passing a function instead of a value, the function takes the current state as the parameter and returns the new state.

Function state changes are required when the new state depends on the old state.

You need to set the new to-dos based on the current to-dos. This situation means you should use the function version of the setter. In this function, first lookup the index, i, and then mark the Todo at that index as done.

Finally, you need to trigger the function when a user clicks the check box to tie this all together.

<TodoListItem key={} onChange={() => onComplete(} {...todo} />

And now, when you click the checkbox, it... checks and unchecks the box. Time to filter those out!

Filter out the done results

The last step is to hide the completed list items. You can do this with a simple filter before mapping the list items to the DOM. Add in this line before the .map line:

.filter(({ done }) => !done)

This code destructures the to-do and pulls out the done attribute. You want only to show items that are not done, so true list items should be removed. Use the ! (not) operator to inverse the boolean and filter those out.

And there you have it! When you click on a checkbox to complete an item, it will disappear and be removed from the list!

Finishing up with style

Your list might not look great with the checkbox. You can add some CSS to clean things up and get it to look great.

ul li {
padding: 0.5rem;
display: flex;
align-items: flex-start;

The above code will use flex to get all the items aligned together.

ul li span {
margin-left: 0.5rem;

The above CSS will give your text some breathing room from the checkbox.

And there you have it! Your to-do app now can add and complete items! Combined with the local storage additions, you can start to use this on a device to track and manage to-dos! But, of course, there is much more to add to make this full-featured and even more helpful.

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