ESLint
===

## Install ESLint

npm install eslint eslint-plugin-react

## Initilize ESLint

npx eslint --init

ESLint will ask you some questions to set up some basic settings:

√ How would you like to use ESLint? · style
√ What type of modules does your project use? · esm
√ Which framework does your project use? · react
√ Does your project use TypeScript? · No / Yes
√ Where does your code run? · browser
√ How would you like to define a style for your project? · prompt
√ What format do you want your config file to be in? · JSON
√ What style of indentation do you use? · 4
√ What quotes do you use for strings? · single
√ What line endings do you use? · windows
√ Do you require semicolons? · No / Yes

After initialization, ESLint will create a config file `.eslintrc` or `.eslintrc.*`.
According to our selection, ESLint creates a file `.eslintrc.json`:

{
    "env": {
        "browser": true,
        "es2021": true
    },
    "extends": [
        "eslint:recommended",
        "plugin:react/recommended"
    ],
    "parserOptions": {
        "ecmaFeatures": {
            "jsx": true
        },
        "ecmaVersion": 12,
        "sourceType": "module"
    },
    "plugins": [
        "react"
    ],
    "rules": {
        "indent": [
            "error",
            4
        ],
        "linebreak-style": [
            "error",
            "windows"
        ],
        "quotes": [
            "error",
            "single"
        ],
        "semi": [
            "error",
            "always"
        ]
    }
}

## Set up ESLint Config
According to the [recommandation](https://github.com/yannickcr/eslint-plugin-react#configuration) by `
eslint-plugin-react `, let's append the following code to `.eslintrc.json`

"settings": {
"react": {
  "createClass": "createReactClass", // Regex for Component Factory to use,
                                     // default to "createReactClass"
  "pragma": "React",  // Pragma to use, default to "React"
  "fragment": "Fragment",  // Fragment to use (may be a property of <pragma>), default to "Fragment"
  "version": "detect", // React version. "detect" automatically picks the version you have installed.
                       // You can also use `16.0`, `16.3`, etc, if you want to override the detected value.
                       // default to latest and warns if missing
                       // It will default to "detect" in the future
  "flowVersion": "0.53" // Flow version
},
"propWrapperFunctions": [
    // The names of any function used to wrap propTypes, e.g. `forbidExtraProps`. If this isn't set, any propTypes wrapped in a function will be skipped.
    "forbidExtraProps",
    {"property": "freeze", "object": "Object"},
    {"property": "myFavoriteWrapper"}
],
"linkComponents": [
  // Components used as alternatives to <a> for linking, eg. <Link to={ url } />
  "Hyperlink",
  {"name": "Link", "linkAttribute": "to"}
]

To Change the number of spaces in indent, we can replace the value of `"indent"` by:

"indent": [
    "error",
    2
],

Now our `.eslintrc.json` looks like:

{
    "env": {
        "browser": true,
        "es2021": true
    },
    "extends": [
        "eslint:recommended",
        "plugin:react/recommended"
    ],
    "parserOptions": {
        "ecmaFeatures": {
            "jsx": true
        },
        "ecmaVersion": 12,
        "sourceType": "module"
    },
    "plugins": [
        "react"
    ],
    "rules": {
        "indent": [
            "error",
            2
        ],
        "linebreak-style": [
            "error",
            "windows"
        ],
        "quotes": [
            "error",
            "single"
        ],
        "semi": [
            "error",
            "always"
        ]
    },
    "settings": {
        "react": {
          "createClass": "createReactClass", // Regex for Component Factory to use,
                                             // default to "createReactClass"
          "pragma": "React",  // Pragma to use, default to "React"
          "fragment": "Fragment",  // Fragment to use (may be a property of <pragma>), default to "Fragment"
          "version": "detect", // React version. "detect" automatically picks the version you have installed.
                               // You can also use `16.0`, `16.3`, etc, if you want to override the detected value.
                               // default to latest and warns if missing
                               // It will default to "detect" in the future
          "flowVersion": "0.53" // Flow version
        },
        "propWrapperFunctions": [
            // The names of any function used to wrap propTypes, e.g. `forbidExtraProps`. If this isn't set, any propTypes wrapped in a function will be skipped.
            "forbidExtraProps",
            {"property": "freeze", "object": "Object"},
            {"property": "myFavoriteWrapper"}
        ],
        "linkComponents": [
          // Components used as alternatives to <a> for linking, eg. <Link to={ url } />
          "Hyperlink",
          {"name": "Link", "linkAttribute": "to"}
        ]
    }
}

## Run ESLint

npx eslint src/

Message:

C:\Users\lab\react-hello-world2\src\App.js
   2:40  error  'Link' is defined but never used                no-unused-vars
   8:20  error  Missing semicolon                               semi
  14:1   error  Expected indentation of 8 spaces but found 10   indent
  15:1   error  Expected indentation of 10 spaces but found 12  indent
  16:1   error  Expected indentation of 8 spaces but found 10   indent

✖ 5 problems (5 errors, 0 warnings)
  4 errors and 0 warnings potentially fixable with the `--fix` option.

Check the file `C:\Users\lab\react-hello-world2\src\App.js`:

import React, { Component } from 'react';
import { BrowserRouter, Switch, Route, Link } from 'react-router-dom';
import HelloWorld from './helloWorld';

class App extends Component {
  constructor(props) {
    super(props);
    this.state = {}
  }

  render() {
    return (
      <BrowserRouter>
          <Switch>
            <Route exact path='/' component={HelloWorld} />
          </Switch> 
      </BrowserRouter>
    );
  }
}

export default App;

## Fix errors
### Fix Problems Automatically

 npx eslint src/ --fix

Message:

C:\Users\lab\react-hello-world2\src\App.js
  2:40  error  'Link' is defined but never used  no-unused-vars

✖ 1 problem (1 error, 0 warnings)

Check again the file `C:\Users\lab\react-hello-world2\src\App.js`:

import React, { Component } from 'react';
import { BrowserRouter, Switch, Route, Link } from 'react-router-dom';
import HelloWorld from './helloWorld';

class App extends Component {
  constructor(props) {
    super(props);
    this.state = {};
  }

  render() {
    return (
      <BrowserRouter>
        <Switch>
          <Route exact path='/' component={HelloWorld} />
        </Switch> 
      </BrowserRouter>
    );
  }
}

export default App;

We can find that there is still a problem, let's manually fix it.

### Fix Problem Manually
Remove `, Link` in the line 2 in `C:\Users\lab\react-hello-world2\src\App.js`.

Run ESLint again, we can find that there is no ouput message, which means there is no errors.