Now that we have a UI that is purely driven by the state of our app, we need to add functionality to allow the UI to drive the state. This is often done by creating functions that call setState like we saw in the TodoHeader. Values from the state are then passed down to the UI as props.

We’ll be learning in part 2 of this workshop how we can expose these functions without explicitly passing them down via props.

This is our core “business logic” and handles everything our basic “CRUD” operations: Create, Read, Update, Delete. We don’t have time to walk through writing all of those functions, but you can see that they are already provided in the demo’s TodoApp and passed into our components.

Intro to TypeScript

Taking a look at our components in TodoApp, you can see that our list of props is not just getting longer, but is getting much more complex! We’re passing through functions with various signatures, complex todos objects, and filter strings which are always one of three values.

As applications grow, it becomes increasing difficult to remember what each function does, or what each todo contains. Also, as JavaScript is a loosely typed language, if I wanted to change the value of todos to an array inside my TodoList, JavaScript wouldn’t care. But if TodoListItems was expecting an object, our application would break.

It for these two reasons that the entire industry is shifting to writing applications that are strongly typed, and many are using TypeScript to accomplish that.
As TypeScript’s website states:

TypeScript is a superset of JavaScript that compiles to plain JavaScript.

If you’ve ever used Sass you are familiar with this concept. In the same way that all valid CSS is valid Sass, all valid JavaScript is valid TypeScript. That’s why most of these exercises have been written in ts and tsx files instead of js and jsx files.

Let’s dive into the demo and see how TypeScript can help us better understand our component props and guard against future regressions.

Demo

Let’s start off in the TodoList, as that has the most data flow up and down. There isn’t any interactive UI in this component, as we’re simply passing completed down to each TodoListItem, but we can write a props interface for the component to make sure that everything gets passed down properly.

Writing TodoListProps

Looking at our TodoApp we know that TodoList has three props: filter, todos, and complete. We’ll start by creating an interface called TodoListProps that represents this component’s props.

interface TodoListProps {
  filter: any;
  todos: any;
  complete: any;
}

Note that we’re using the any keyword for now. This won’t give us any type safety, but it does let us specify valid prop names we can pass to this component.

With that interface written, we’ll add it to our component class.

export class TodoList extends React.Component<TodoListProps, any>

Note that the first value in <> is for a props interface, and the second is for state.

Now that we have a typed component, let’s go back to our TodoApp and see what happens if we try to change the name of a prop.

Adding type safety

So far we’ve only established what our prop names are, not the type of values inside of them. Let’s first look at filter and see how we can improve that prop’s type safety.

Filter Type

We know that filter shouldn’t be an object, array or function, so we can specify it should always be a string like this:

interface TodoListProps {
  filter: string;
  todos: any;
  complete: any;
}

But since we know that the filter can be only one of three values, we can make that explicit with a union type:

interface TodoListProps {
  filter: 'all' | 'active' | 'completed';
  todos: any;
  complete: any;
}

Now try going back to TodoApp and changing the filter attribute in TodoList to something else. You’ll see an error in the editor (if using VS Code) and on the command line when you save the file.

Complete Type

The complete prop isn’t data, but a function. Fortunately, TypeScript can handle function types just as well as data.

interface TodoListProps {
  filter: 'all' | 'active' | 'completed';
  todos: any;
  complete: (id: string) => void;
}

For functions we are only concerned with the parameters passed in and the return value. You can see in the example above that the function takes in an id of type string and returns void, which means it has no returned value.

Technically, all functions in JavaScript return undefined if no other return value is specified, but declaring a return type of void causes TypeScript to error if you try to return a value from the function (or use its default returned value of undefined).

Todos Type

The todos prop is interesting in that todos is an object with a bunch of unknown keys. So here’s what that interface would look like.

interface TodoListProps {
  filter: 'all' | 'active' | 'completed';
  todos: {
    [id: string]: {
      label: string;
      completed: boolean;
    };
  };
  complete: (id: string) => void;
}

Note that [id: string] does not indicate an array; it is an object index signature.

Now that our interface is complete, try changing the word “all” in filter === all and see that VS Code will tell you this condition will always be false. Compare this to plain JavaScript: if you had a typo in that line, you wouldn’t understand why your filter wasn’t working.

Sharing types

Most of our components will need to specify types for todos and filter, so it’s a good thing that TypeScript allows us to share types between files. I’ve already written up and exported those shared types in the file TodoApp.types.ts, so we just need to import them and use them in our interface.

import { FilterTypes, Todos } from '../TodoApp.types';
interface TodoListProps {
  complete: (id: string) => void;
  todos: Todos;
  filter: FilterTypes;
}

Updating TodoApp

Our TodoApp doesn’t take any props, but it does have state. We can use TypeScript to define that as well.

I’ve already imported Todos and FilterTypes into the TodoApp, so we just need to add them to our class. If we want, we can even skip a separate interface definition and just declare the type inline. (This is not recommended for types of any complexity or types that are used in multiple places.)

export class TodoApp extends React.Component<{}, { todos: Todos; filter: FilterTypes }>

Note that the first value in <> always refers to props. Since TodoApp takes none, we’ll set it to an empty object type.

Writing TodoListItemProps

Jumping down to the TodoListItem, as we start to write the TodoListItemProps we realize that two of the props, label and completed, have already been defined in the TodoItem interface in TodoApp.types. So we can make TodoListItemProps reuse the TodoItem interface by extending it.

interface TodoListItemProps extends TodoItem {
  id: string;
  complete: (id: string) => void;
}

The end result of this is an interface with all four properties: id, complete, completed and label.

Next we can pull in the remaining props in the render function:

const { label, completed, complete, id } = this.props;

And then use the input’s onChange event to fire our complete callback. We can see in the signature that complete expects an id of type string, so we’ll pass our id prop in.

A callback function is a function passed into a component as a prop.

<input type="checkbox" checked={completed} onChange={() => complete(id)} />

Note that the function param and prop name just happen to be the same. This isn’t required.

Now that our todos are firing the onChange callback, give them a click and take look at how the app responds. Since our footer text is based on the number of unchecked todos, the footer will automatically update to reflect the new state.

Exercise

If you don’t already have the app running, start it by running npm start from the root of the frontend-bootcamp folder. Click the “exercise” link under day 1 step 7 to see results.

TodoFooter

1. Open TodoFooter and write a TodoFooterProps interface. It should include two values, a function and an object. Use this interface in the function props like this: (props: TodoFooterProps)
2. Write an _onClick function that calls props.clear.

   Since TodoFooter is not a class, the _onClick function needs to be stored in a const placed before the return.

1. Assign _onClick to the button’s onClick prop. You won’t need to use this since the component isn’t a class.
2. Test out this functionality. Check a few todos complete and click the Clear Completed button.

TodoHeader

1. Open TodoHeader and write TodoHeaderProps which will include three values. Replace the first any in the class declaration with this interface.
2. This component also has state. Write TodoHeaderState (there’s just one value), and add this where the second any was.
3. Add _onFilter to each of the filter buttons

   Note that we can’t add new parameters to onClick, but we can pull information from the event target!

1. Write an _onAdd method that calls addTodo on the current labelInput, then sets the labelInput in state to an empty string
2. Call _onAdd from the submit button
3. Check out this new functionality! We can now add and filter todos!