frontend-bootcamp/step1-07/demo/README.md

Creating a UI Driven State

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, that are 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 of "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 as well as 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 type 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 is because of these two reasons that the entire industry is shifting to writing applications that are strongly typed, and are using TypeScript to accomplish that.

As their website state:

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 say that all valid CSS is valid Sass, all valid JavaScript is valid TypeScript. That's why most of this project has been writing 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 actionable UI in this component as we're simply passing completed down to each TodoListItem, but we can write a component interface 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 filter. We'll start by creating and interface that represents this component's props called TodoListProps.

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 innumerate the valid props 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 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 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 explicitly write it that way with union types:

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.

Complete Type

The complete props isn't data, but rather 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 return.

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 the [] notation does not mean an array, it is a computed property notation.

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. Imagine you had a typo in that line and you couldn't understand why your filter wasn't working.

Abstracting types

Most of our components are going to need to add types for todos and filter, so it's a good thing that TypeScript allows us to abstract those. I've already written up and exported those shared types in the file TodoApp.types.ts, so we just need to import them and pull them into 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. We can even skip the 'interface', if we want to, and add them directly to the class.

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.

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 in the same way we can reuse individual types (FilterTypes), we can reuse, and extend upon entire interfaces.

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

The end result of this is an interface with all 4 properties, id, complete, completed and label.

Next we can pull in the remaining props:

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 we expect and 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 response. Since our footer text is driven off of the number of unchecked todos, the footer will automatically update to reflect the new state.