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# Notes-04-05-eslint-and-babel

### Eslint <a href="#eslint" id="eslint"></a>

* add `env` entry for different global environment variables, and module system
* add for each rule, basic switches are `off`, `warn` issue with `exit(0)` , `error` issue with `exit(1)`

**How eslint deal with jsx**

1. `parserOptions.ecmaFeatures.jsx: true`, parser(**espree**) will parse `jsx` to `JSXIdentifier` Node in `ast`
2. `react/react-in-jsx-scope` rule used to check: when there is jsx in file, there should be a `React` in variable

**Gotchas**

Til now(04-05-2020), `eslint` does not deal with `jsx` in a parsed way. That been said, `eslint` treat `jsx` as **raw** `jsx` syntax, and use `react-in-jsx-scope` to trick the way around to check if `React` exist.

The proper way, as `babel` did, parsing `jsx` to `React.createElement()`, **providing proper/standard variable referencing**, is not supported by `babel-eslint <= v10.x`. Since it parse the code with a set of default opts, **without** being able to access the project's `.babalrc` config file.

Tricky part is, github default page for `babel-eslint` is actually for **UNRELEASED v11.x**, in which a new config field `parserOption.babelOptions` is added. With `babel` changed to a peerDep and external parse config.

> This ensures that the same Babel configuration is used during both linting and compilation.

To enable different `jsx` transpiler for `eslint`, there are 3 ways

1. `react/react-in-jsx-scope` can be worked around with [**jsx pragma**](https://github.com/yannickcr/eslint-plugin-react/blob/80f3826dd854796d25244ad34782b9147b90db1d/lib/util/pragma.js#L45), it can be added as `/** jsx NewReact.createEl */`, or add pragma as [shared settings](https://eslint.org/docs/user-guide/configuring#adding-shared-settings)
2. In `preact`, the recommended way is to use `alias` in bundler(webpack/parcel/browersify), and the `jsx` code can easily share same support with react **before bundle**.
3. Use `babel-eslint >= v11.x` and use a shared `.babelrc`

### Babel <a href="#babel" id="babel"></a>

> For compatibility reasons, `.babelrc` is an alias for `.babelrc.json`

Use `babel.config.js` is a recommended way to do config cos it's more flexible, while cache is harder so explicit `api.cache()` setup is forced


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