# Linter

A linter is the equivalent of a grammar check for code. It doesn't mean the code is correct, but it helps maintain consistency across the code base.

For this project, we will be using [TSLint](https://palantir.github.io/tslint/) for Typescript.

Let's start by adding `typescript` and `tslint` and creating a `tslint.json` file at the root of the project.

```
$ yarn add tslint typescript
$ tslint --init
```

Create a file in `packages/api/index.ts` with invalid code structure:

{% code title="packages/api/index.ts " %}

```typescript
var test = function () {
    return 'hello world'
}
```

{% endcode %}

```
$ tslint packages/api/index.ts

ERROR: packages/api/index.ts:1:1 - Forbidden 'var' keyword, use 'let' or 'const' instead
ERROR: packages/api/index.ts:1:12 - non-arrow functions are forbidden
ERROR: packages/api/index.ts:1:20 - Spaces before function parens are disallowed
ERROR: packages/api/index.ts:2:12 - ' should be "
ERROR: packages/api/index.ts:2:25 - Missing semicolon
ERROR: packages/api/index.ts:3:2 - file should end with a newline
ERROR: packages/api/index.ts:3:2 - Missing semicolon
```

Have a look at [all the available rules](https://palantir.github.io/tslint/rules/).

### Project specific configuration

The Node API and React app `tslint.json` configuration could be different. We can share common rules between the project from the root.

```
$ cd packages/api
yarn add tslint typescript
tslint --init
```

Replace the content of `packages/api/tslint.json` with:

{% code title="packages/api/tslint.json" %}

```typescript
{
    "extends": ["../../tslint.json"]
}

```

{% endcode %}

You can do the same with `web` but we'll come back to it later.

{% hint style="info" %}
[`linter`](https://github.com/florianherrengt/book-code/tree/linter) branch available on GitHub.
{% endhint %}