# Configuring utilities for the `APPSYNC_JS` runtime
<a name="configure-utilities"></a>

AWS AppSync provides the following two libraries that help you develop event handlers with the `APPSYNC_JS` runtime: 
+ `@aws-appsync/eslint-plugin` - Catches and fixes problems quickly during development.
+ `@aws-appsync/utils` - Provides type validation and autocompletion in code editors.

## Configuring the eslint plugin
<a name="configure-eslint-plugin"></a>

[ESLint](https://eslint.org/) is a tool that statically analyzes your code to quickly find problems. You can run ESLint as part of your continuous integration pipeline. `@aws-appsync/eslint-plugin` is an ESLint plugin that catches invalid syntax in your code when leveraging the `APPSYNC_JS` runtime. The plugin allows you to quickly get feedback about your code during development without having to push your changes to the cloud.

`@aws-appsync/eslint-plugin` provides two rule sets that you can use during development. 

**"plugin:@aws-appsync/base"** configures a base set of rules that you can leverage in your project. The following table describes these rules.


| Rule | Description | 
| --- | --- | 
| no-async | Async processes and promises are not supported. | 
| no-await | Async processes and promises are not supported. | 
| no-classes | Classes are not supported. | 
| no-for | for is not supported (except for for-in and for-of, which are supported) | 
| no-continue | continue is not supported. | 
| no-generators | Generators are not supported. | 
| no-yield | yield is not supported. | 
| no-labels | Labels are not supported. | 
| no-this | this keyword is not supported. | 
| no-try | Try/catch structure is not supported. | 
| no-while | While loops are not supported. | 
| no-disallowed-unary-operators | \$1\$1, --, and \$1 unary operators are not allowed. | 
| no-disallowed-binary-operators | The instanceof operator is not allowed. | 
| no-promise | Async processes and promises are not supported. | 

**"plugin:@aws-appsync/recommended"** provides some additional rules but also requires you to add TypeScript configurations to your project.


| Rule | Description | 
| --- | --- | 
| no-recursion | Recursive function calls are not allowed. | 
| no-disallowed-methods | Some methods are not allowed. See the [Runtime features](runtime-features-overview.md) for a full set of supported built-in functions. | 
| no-function-passing | Passing functions as function arguments to functions is not allowed. | 
| no-function-reassign | Functions cannot be reassigned. | 
| no-function-return | Functions cannot be the return value of functions. | 

To add the plugin to your project, follow the installation and usage steps at [Getting Started with ESLint](https://eslint.org/docs/latest/user-guide/getting-started#installation-and-usage). Then, install the [plugin](https://www.npmjs.com/package/@aws-appsync/eslint-plugin) in your project using your project package manager (e.g., npm, yarn, or pnpm):

```
$ npm install @aws-appsync/eslint-plugin
```

In your `.eslintrc.{js,yml,json}` file, add **"plugin:@aws-appsync/base"** or **"plugin:@aws-appsync/recommended"** to the `extends` property. The snippet below is a basic sample `.eslintrc` configuration for JavaScript: 

```
{
  "extends": ["plugin:@aws-appsync/base"]
}
```

To use the **"plugin:@aws-appsync/recommended"** rule set, install the required dependency:

```
$ npm install -D @typescript-eslint/parser
```

Then, create an `.eslintrc.js` file:

```
{
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "ecmaVersion": 2018,
    "project": "./tsconfig.json"
  },
  "extends": ["plugin:@aws-appsync/recommended"]
}
```