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.

Static Analysis Testing JavaScript Applications

Use pre-built ESLint Configuration using extends

Instructor: [00:00] As cool as it is that ESLint is so customizable that I can configure every single one of the rules, ESLint has a lot of rules, like a ton of built-in rules. There're even rules that you can install from third-party packages.

[00:14] I don't want to have to configure every single one of these rules. It'd be really nice if somebody just gave me a configuration that was like a recommended configuration.

[00:23] ESLint does have that. Instead of configuring each one of these rules, we can say `"extends"`, and specify a rule set that we want to extend. ESLint ships with a rule set, and it's called `"eslint:recommended"`.

### .eslintrc
```json
{
  "parserOptions": {
    "ecmaVersion": 2019,
    "sourceType": "module",
    "ecmaFeatures": {
      "jsx": true
    }
  },
  "extends": ["eslint:recommended"],
  "rules": {
    "strict": ["error", "never"],
    "valid-typeof": "error",
    "no-unsafe-negation": "error",
    "no-unused-vars": "error",
    "no-unexpected-multiline": "error",
    "no-undef": "error"
  },
  "env": {
    "browser": true
  }
}
```

[00:38] With that, we don't need any of these rules here, except for the `"strict"` rule here. We want to override the recommended configuration because, in our case, we never want to have `'use strict'`. We'll leave that one in there. Our rule configuration that we specify in here will override anything that's in the extends array here.

### .eslintrc
```json
{
  "parserOptions": {
    "ecmaVersion": 2019,
    "sourceType": "module",
    "ecmaFeatures": {
      "jsx": true
    }
  },
  "extends": ["eslint:recommended"],
  "rules": {
    "strict": ["error", "never"]
  },
  "env": {
    "browser": true
  }
}
```

[00:58] If we wanted to extend multiple configurations, we just add more to the array here. The one that comes last will be the one that overrides the previous. That's how the configuration is merged across these different configurations.

### .eslintrc
```json
{
  "parserOptions": {
    "ecmaVersion": 2019,
    "sourceType": "module",
    "ecmaFeatures": {
      "jsx": true
    }
  },
  "extends": ["eslint:recommended", "...", "...", "...", "..."],
  "rules": {
    "strict": ["error", "never"]
  },
  "env": {
    "browser": true
  }
}
```

[01:11] With that, I can go ahead and save. Look at my example, and I'm still getting all of the same warnings that I was getting before except I didn't have to configure each one of these rules manually myself.

[01:23] Let's go ahead and we'll fix all these. I'll run `npx eslint . --fix`. You get all those auto fixes in there. Then we'll manually fix a couple of these other ones. That's going to be `'string'`. Let's get rid of those extra lines.

### example.js
```js
const username = 'freddy'
typeof username === 'string'

if (!('serviceWorker' in navigator)) {
  // you have an old browser :-(
}

const greeting = 'hello'
console.log('${greeting} world!')
[1, 2, 3].forEach(x => console.log(x))
```

[01:36] This one needs to be a template literal, that takes care of that one. This one needs just a `;` in front of it, so we make sure we avoid that unexpected multi-line. With that, now our code is looking much better and our configuration is looking better as well.

### example.js
```js
const username = 'freddy'
typeof username === 'string'

if (!('serviceWorker' in navigator)) {
  // you have an old browser :-(
}

const greeting = 'hello'
console.log(`${greeting} world!`)
;[1, 2, 3].forEach(x => console.log(x))
```

[01:50] In review, all that we did here was we used ESLint's built-in recommended configuration. Then we could override some of those rules ourselves in our own custom configuration.


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

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

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.

Validate All Files are Properly Formatted with 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.

Use pre-built ESLint Configuration using extends

Transcript

Use pre-built ESLint Configuration using extends