[00:00] We've been talking about declaration files so far, but we've not really covered their actual main use case. So inside here, we have an index.ts file in which we're importing myfunc from example, and we are then calling myfunc here. But we're getting a big old error from here saying,

[00:18] could not find a declaration file for this module that we're trying to import from because it's a JS file. And this sometimes happens, you'll sometimes have JS files in your TypeScript application. And notice that the error is saying, could not find a declaration file for it.

[00:37] So your job is to try to use a declaration file to essentially type this or describe the exports of this example.js file. This is really what declaration files are for. And so you're gonna find, I think some pretty easy docs links to get you there. Good luck.


Let's shift gears to look at the practical use case of declaration files.

Consider this `example.js` JavaScript file that exports `myFunc`:

```javascript
// example.js
export const myFunc = () => {
  return "Hello World!";
};
```

The `myFunc` function is then imported inside of a TypeScript `index.ts` file:

```tsx
// index.ts
import { myFunc } from "./example"; // red squiggly line under ./example

myFunc();
```

However, there is an error in the import statement because TypeScript expects a declaration file for this JavaScript module:

```tsx
// hovering over the error shows:
Could not find a declaration file for module './example'.
```

## Challenge

Your challenge is to create a declaration file for the `example.js` file.

This file will describe the exports of `example.js` and help TypeScript understand the structure of the JavaScript module you're using.



Learn how to integrate JavaScript files into your TypeScript projects using declaration files.

[00:00] Okay, so let's create a declaration file that describes the contents of example.js. Now TypeScript won't by default try to go into this .js file and try to figure out what's in it and then figure out the modules.

[00:16] It needs a declaration file to basically say this is what is in this .js file. So we're going to do that. We're going to go into this file and we're going to create an example.d.ts. It has to have exactly the same name, exactly the same casing. And I'll go back to my index.ts file to figure this out.

[00:34] So because the thing that we're trying to describe is essentially a module, we also need our example.d.ts file to be a module. So let's add an export empty object there. Now this takes a little while to update, I think, and it might not even update.

[00:49] So what I tend to do is I tend to just take a look at this and re-add the import just so it catches up. So now we're getting an error saying module example has no exported member myfunc.

[01:01] So even though the .js file has something called myfunc exported from it, we're not actually describing that in the .d.ts file. So how do we do that? Well, we can say export function and then export function myfunc. And that is going to be a function that returns, what does it return?

[01:19] It just returns a string here. And so export function myfunc, this is going to just return string. Copilot's wrong in this instance. And now you notice that inside our index.ts file, myfunc is just a function that returns a string. Beautiful.

[01:35] Now this is so, so nice because it means now that although it won't stay in sync with our .js, so if we make it return a number, we'll also have to go into this and say that myfunc returns a number too. This means that the .d.ts gets to just sit next to the .js file and describe what's in it.

[01:53] And you notice too that inside the source directory, it kind of gets grouped together. So you can see that the source here, it now has example.js and the example.d.ts, because they're related, get nested together. So this is a really nice way to basically describe JavaScript files in your project that, for instance,

[02:12] maybe were auto-generated by script or, let's say, haven't been migrated across the TypeScript yet. And what we'll see is that this pattern is basically how you ship JavaScript files in NPM while still having them be compatible with your TypeScript project.


TypeScript doesn't automatically interpret the contents of JavaScript files. Instead, it needs a declaration file to describe the contents of the JavaScript file.

Here's our existing JavaScript module, that exports a function `myFunc`:

```js
// example.js
export const myFunc = () => {
  return "Hello World!";
};
```

In order to create the declaration file, we first need to create a file with the same name and casing as the JavaScript file. In this case, `example.d.ts`.

The first thing we'll do inside the file is add and empty `export {};` which tells TypeScript to treat the file as a module.


Next, we need to export the same function as the JavaScript file, including the return type:

```tsx
// example.d.ts
export function myFunc(): string;

export {};
```

With this newly created type definition file, we can now import and use the JavaScript module in our TypeScript's `index.ts` file without errors:

```tsx
import { myFunc } from "./example";

myFunc();
```

This is the same approach you would follow for any JavaScript module you want to use in your TypeScript project.


Learn how to create declaration files for JavaScript modules to integrate them into TypeScript projects.

Typing a JavaScript Module

Using Declaration Files with JavaScript in TypeScript

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Rest Parameters in TypeScript

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The `void` Type

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Using Declaration Files with JavaScript in TypeScript

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