1. You can’t force everyone on your project to use the prettier integration for their editor, so let’s add a validation script to verify that prettier has been run on all project files.
2. Fortunately for us, Prettier accepts a `--list-different` flag that you can use that will throw an error when code is not up to the standard of your project.

Static Analysis Testing JavaScript Applications

Validate All Files are Properly Formatted with Prettier

Instructor: [00:00] One thing that I find quite helpful in projects is to have a `"script"` that validates that the project is in a good state. I'm going to have a `"validate"` script here. I guess I need to run the build and linting, so I'll run `npm run lint`, and then `npm run build`. With that, I can run `npm run validate`. That runs all the linting, runs all the building. It makes sure that this project is in a good state.

### package.json
```json
{
  ...
  "scripts": {
    "build": "babel src --out-dir dist",
    "lint": "eslint --ignore-path .gitignore .",
    "format": "prettier --ignore-path .gitignore --write \"**/*.+(js|json)\"",
    "validate": "npm run lint && npm run build"
  }
}
```

### Terminal Input
```
npm run validate
```

[00:26] I also want to make sure that the files in the project have been properly formatted. To do that, Prettier exposes a mechanism for that. I can say `"check-format"`. We'll basically copy this same thing, put it right here. Instead of `--write`, we're going to do `--list-different`.

### package.json
```json
{
  ...
  "scripts": {
    "build": "babel src --out-dir dist",
    "lint": "eslint --ignore-path .gitignore .",
    "format": "prettier --ignore-path .gitignore --write \"**/*.+(js|json)\"",
    "check-format": "prettier --ignore-path .gitignore --list-different \"**/*.+(js|json)\"",
    "validate": "npm run lint && npm run build"
  }
}
```

[00:49] For this, I'm going to quickly disable `formatOnSave` so we can see what this looks like. If I go to my `example.js` here, and we just make some formatting mistake, then we go to our terminal here. We'll run `npm run check format`.

### Terminal
![npm run check format](https://res.cloudinary.com/dg3gyk0gu/image/upload/v1574890640/transcript-images/javascript-validate-all-files-are-properly-formatted-npm-run-check-format.jpg)

[01:07] Prettier is going to go through all of the files that we've given it, and it'll check if I were to format this file, would that actually change. This is nice, and I can add it to my `"validate"` script here, so `npm run check format`.

### package.json
```json
{
  ...
  "scripts": {
    "build": "babel src --out-dir dist",
    "lint": "eslint --ignore-path .gitignore .",
    "format": "prettier --ignore-path .gitignore --write \"**/*.+(js|json)\"",
    "check-format": "prettier --ignore-path .gitignore --list-different \"**/*.+(js|json)\"",
    "validate": "npm run check--format && npm run lint && npm run build"
  }
}
```

[01:22] If I run `npm run validate`, that's going to run all those scripts. It'll run the `prettier` one first, and we'll see that we didn't format. That way, we can run `npm run format`, and then `npm run validate` so we can know that this project is in a good state when that `"validate"` script passes.

### Terminal Input
```
npm run format
...
npm run validate
```

[01:42] One thing that I don't like about this is that `"check-format"` script has so much duplication between these two. What I'm going to do is I'm going to add a `prettier` script right here. This is going to have all of the commonalities between these two scripts.

[01:55] Let's get rid of the `--write` here, because that's different. Then in this format script, I'm going to run `npm run prettier`, and then add a `--`. That tells npm to forward all the remaining arguments to this script that I'm calling here, the remaining argument being `--write`.

### package.json
```json
{
  ...
  "scripts": {
    "build": "babel src --out-dir dist",
    "lint": "eslint --ignore-path .gitignore .",
    "prettier": "prettier --ignore-path .gitignore \"**/*.+(js|json)\"",
    "format": "npm run prettier -- --write",
    "check-format": "prettier --ignore-path .gitignore --list-different \"**/*.+(js|json)\"",
    "validate": "npm run lint && npm run build"
  }
}
```

[02:16] We'll do the same thing for our check format script. Let's just copy this. We'll paste that here, and instead of `--write`, we'll do `--list-different`. That way, we can have all of our general Prettier stuff in this one `"prettier"` script, and then just forward on special flags for the `"format"` and `"checkpformat"` scripts.

### package.json
```json
{
  ...
  "scripts": {
    "build": "babel src --out-dir dist",
    "lint": "eslint --ignore-path .gitignore .",
    "prettier": "prettier --ignore-path .gitignore \"**/*.+(js|json)\"",
    "format": "npm run prettier -- --write",
    "check-format": "npm run prettier -- --list-different",
    "validate": "npm run lint && npm run build"
  }
}
```

[02:37] With that, I can run `npm run validate` and I have pretty good confidence that this project is in a good state, because the `"validate"` script passed.

### Terminal Input
```
npm run validate
```

[02:45] In review, to do all this, we created a new `"validate"` script that runs this new `"check-format"` script, our `"lint"` script, and our build. We made the `"check-format"` script by making a new `"prettier"` script, which contains the `prettier --ignore-path` and the files that we want to run `"prettier"` against.

[03:02] Then we updated our `"format"` script to run that `"prettier"` script with `--write`. Then our `"check-format"` script runs that `"prettier"` script with `--list-different`.


There are a ton of ways your application can break. One of the most common sources of bugs is related to typos and incorrect types. Passing a string to a function that expects a number, or falling prey to a common typo in a logical statement are silly mistakes that should never be made, but this happens all the time.

We could write a comprehensive suite of automated tests for our entire codebase to make certain mistakes like this never happen, but that would likely be too much work and slow development down to be worth the benefit. Luckily for us, there are tools like ESLint, TypeScript, Prettier, and more which we can use to satisfy a whole category of testing with a great developer experience.

Intro to Static Analysis Testing JavaScript Applications

The static code analysis and linting tool ESLint is the de-facto standard for linting JavaScript projects. In this lesson we’ll see how to install, run, and configure it for your preferences.

Lint JavaScript by Configuring and Running ESLint

Whatever editor you use, chances are there’s probably some plugin or extension you can use to integrate ESLint and your editor to get a nice in-editor experience using ESLint so you don’t have to run the ESLint script to check your code and instead can identify issues as you’re writing and editing your code. Let’s take a look at what you can do with [the ESLint Extension for VSCode](https://marketplace.visualstudio.com/items?itemName=dbaeumer.vscode-eslint).

Use the ESLint Extension for VSCode

ESLint has [a TON of rules](https://eslint.org/docs/rules/) you can configure for your project. But who has time to do all that? This is why ESLint has an extends configuration which you can use to base your own configuration off of other configurations. Let’s add all of ESLint’s built-in recommended rules to our project and override just the ones we want to change.

Use pre-built ESLint Configuration using extends

Let’s add an npm script called lint so we can run ESLint whenever we want with the proper command line flags.

Run ESLint with npm Scripts

The code formatting tool [prettier](https://prettier.io/) can help you avoid a lot of useless time spent formatting code and arguing about code formatting with your co-workers. It can also help you catch subtle issues with your code that you may not notice otherwise. In this lesson we’ll learn how to install and run prettier.

Format Code by Installing and Running Prettier

1. Prettier is a pretty opinionated tool, but it does allow for some customization. In this lesson we’ll check out [the prettier playground](https://prettier.io/playground/) and see what options we want to enable in our project’s `.prettierrc` file. 
2. After adding in our custom configuration, we’ll create a `.prettierignore` file so that you can avoid formatting any files generated within the project such as `node_modules` or a `build` folder.

Configure Prettier

Running prettier manually is great, but it’d be even better if we could just run prettier anytime we save our file. Whatever editor you use, there’s likely a plugin or extension for it to integrate with prettier. Let’s try out [the Prettier Extension for VSCode](https://marketplace.visualstudio.com/items?itemName=esbenp.prettier-vscode).

Use the Prettier Extension for VSCode

Because prettier can automatically fix a lot of stylistic issues in our codebase, it’s not necessary to have eslint check for those and it can actually be kind of annoying if it does. So let’s see how we can use eslint-config-prettier to disable all rules that are made irrelevant thanks to prettier.

Disable Unnecessary ESLint Stylistic Rules with eslint-config-prettier

ESLint can check for a lot of things, but it’s not a great tool for checking the types of variables that flow through your application. For this you’ll need a type-checking tool like [Flow](https://flow.org/) or [TypeScript](https://www.typescriptlang.org/). Let’s see how we can configure our project to work with TypeScript.

Avoid Common Errors by Installing and Configuring TypeScript

ESLint does not have support for TypeScript out of the box, but luckily adding support for TypeScript is straightforward thanks to the [TypeScript-ESLint](https://typescript-eslint.io/) project. And now that we have TypeScript, there are a lot of rules that we don’t need ESLint to bother checking because TypeScript will prevent those problems in the first place! Let’s get these two tools integrated.

Make ESLint Support TypeScript Files

We have a few checks we’ll run in continuous integration when someone opens a pull request, but it’d be even better if we could run some of those checks before they even commit their code so they can fix it right away rather than waiting for CI to run. Let’s use husky’s precommit script to run our validation.

Validate Code in a pre-commit git Hook with husky

Rather than failing when a developer has failed to format their files or run linting on all the files in the project on every commit, it would be even better to just automatically format the files on commit and only check the relevant files with eslint. Let’s use lint-staged to run scripts on the files that are going to be committed as part of our precommit hook.

Auto-format All Files and Validate Relevant Files in a precommit Script with lint-staged

I use npm scripts a lot, and that validate script is a great way to bring everything together. But with the way these tools work, we can run them all at the same time and things will work just as well. This will speed up our script runtime a lot, so let’s use [npm-run-all](https://npm.im/npm-run-all) to make that happen.

Validate All Files are Properly Formatted with Prettier

Transcript

Validate All Files are Properly Formatted with Prettier