[00:00] In this exercise, we're starting from where we left off from the previous exercise. But this one, we're trying to make our development process a bit better. The way it's currently working is we're basically having to every time we make a change inside here, we're having to run TSC in order to get the new JavaScript file out.

[00:17] Then we can open it in the browser and check that what we've got is real. So if we say hello world inside here, got to run TSC again, then I go inside here and it says hello world. Great. It would be great if we could automate one of the parts of this, which was running TSC. Your job is to work out if there is a continuous watch mode that we can use with

[00:39] TypeScript in order to make sure that whenever our TypeScript code changes, our JavaScript code also gets emitted. Good luck.


Let's continue from where we left off.

Currently, every time we make a change to our TypeScript file, we have to manually run `tsc` to generate the new JavaScript file.

There's got to be a better way!

## Challenge

Your job is to review the [`tsc` CLI options](https://www.typescriptlang.org/docs/handbook/compiler-options.html) to determine how to automate the TypeScript compilation process.

Currently we have to manually run the TypeScript CLI every time we make a change. The tsc CLI features a watch mode to do this automatically.

[00:00] The solution here is really nice. Inside the proper directory here, I'm going to run tsc-watch. And now, if I look at this, it's saying starting compilation in watch mode. If I open up the example.ts and the example.js, you'll be able to see that when I make changes to the ts,

[00:18] so I say hello world here, it automatically goes through to the example.js here. Now, this tsc watch mode is really, really nice because it's going to check my entire project for me too. So, for instance, I add in, let's say, a wrong thing here, or I say message number, for instance.

[00:36] Then, if I save the file, it's going to catch up with me and it's going to say argument of type string is not assignable to parameter of type number. Look at this. So, this is really nice. Tsc watch gives you, basically, a way of checking your entire project

[00:53] every time you make a change inside there. And so, if I change it back to message string, it's now going to work. And then, if I go over to my browser, I can see that when I refresh, I'm going to see hello world. So, tsc watch is a really critical tool to know about, not only because it means that you get to skip over

[01:11] some parts of running tsc every time, but also because you can use it as a way to monitor the status of your entire project and make sure there are no errors across the whole thing.


Running `tsc --watch` is the solution to automating the TypeScript compilation process.

To see it in action, open the `example.ts` and `example.js` files side by side.

When you make changes to the `example.ts` file, such as adding a "Hello, World!" message, you'll notice that the changes are automatically reflected in the `example.js` file.

If you try to add an incorrect type for a variable (e.g., specifying `message` as a number instead of a string), `tsc` will catch the error and notify you in the terminal as well as in VS Code.

```typescript
const run = (message: number) => {
  console.log(message);
}

run("Hello world!"); // red squiggly line under "Hello world!"
```

```
// inside the terminal

Argument of type 'string' is not assignable to parameter of type 'number'.
run("Hello world!");

Found 1 error. Watching for file changes.
```



Running `tsc` in watch mode is super useful for automatically compiling TypeScript files as you make changes and watching for errors across your entire project.

Running tsc in watch mode allows you to automatically compile TypeScript files as you make changes.

Use tsc's Watch Mode to Automatically Compile TypeScript Files

Automating TypeScript Compilation

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Automating TypeScript Compilation

Challenge

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