Extending an ESLint configuration

Following are the steps to extend an ESLint configuration:

ESLint is now perfectly suited for palindromes.js, but if you try to run it against palindromes.test.js, it won’t go well:
```
$ npx eslint palindromes.test.js

/Users/tburnham/code/test-driven-palindromes/palindromes.test.js
  3:1  error  'describe' is not defined  no-undef
  4:3  error  'it' is not defined        no-undef
  5:5  error  'expect' is not defined    no-undef

✖ 3 problems (3 errors, 0 warnings)
```
All of these problems share the same cause as the module kerfuffle earlier: ESLint doesn’t know that palindromes.test.js will be running in an environment (Jest) where describe, it, and expect are defined as globals.
You could fix the problem with another env entry, but there’s a better way. Jest has an official ESLint plugin, eslint-plugin-jest, which comes with several rules for identifying common test code mistakes. Go ahead and add it to the project:
```
$ npm install --save-dev eslint-plugin-jest@22.1.2
+ eslint-plugin-jest@22.1.2
```
To apply the plugin, you need to make two changes to the ESLint configuration. First, register it in a plugins entry. And second, add its recommended configuration to the extends entry:
```
// .eslintrc.js
module.exports = {
  plugins: ['jest'],
 
```

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