Use Biome in big projects

Biome can provide some tools that can help you to use it properly in big projects, such as monorepo or workspaces that contain multiple projects.

Use multiple configuration files

When you use Biome’s features - either with the CLI or LSP - the tool looks for the nearest configuration file using the current working directory.

If Biome doesn’t find the configuration file there, it starts traversing upwards the directories of the file system, until it finds one.

You can leverage this feature to apply different settings based on the project/folder.

Let’s suppose we have a project that contains a backend app and a frontend app.

Directoryapp
- Directorybackend
  - biome.json
  - package.json
- Directoryfrontend
  - biome.json
  - Directorylegacy-app
    package.json
  - Directorynew-app
    package.json

This means that when you run Biome inside app/backend, Biome will use the configuration file app/backend/biome.json. And when you run from app/frontend/legacy-app or app/frontend/new-app, Biome will use the configuration file app/frontend/biome.json.

Monorepo

Monorepos are particular repositories where multiple packages are stored and maintained in one big repository. Each package can contain its own configuration.

Since v2, Biome supports monorepos out of the box, and you’ll need to set up the project in the following way.

Create a biome.json file at the root of the monorepo. We will use the recommended rules, and customise the formatter options:
biome.json
```
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{
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    "linter": {
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        "enabled": true,
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        "rules": {
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            "recommended": true
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        }
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    },
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    "formatter": {
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        "lineWidth": 120,
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        "indentStyle": "space",
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        "indentWidth": 6,
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    }
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}
```
This file is called the root configuration and it sets the base options inside the project. However, nested configurations can decide to adhere to these options or not. Let’s see how.
Create nested configuration files, one in each package where it is needed. These nested configuration files must have the field "root" set to false. Also, we want these packages to follow the formatting standards set up in the root configuration. In order to so, we will use a new microsyntax available in Biome v2, which is "extends": "//". This syntax tells Biome to extend from the root configuration, regardless of where the nested configuration is.

Let’s create two configuration files, one inside packages/logger and one inside packages/generate. In the former we will disable noConsole, and in packages/generate we will disable the formatter because those are files that are code-generated:
packages/logger/biome.json
```
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{
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    "root": false,
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    "extends": "//",
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    "linter": {
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        "rules": {
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            "suspicious": {
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                "noConsole": "off"
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            }
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        }
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    }
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}
```
packages/generate/biome.json
```
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{
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    "root": false,
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    "extends": "//",
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    "formatter": {
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        "enabled": false
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    }
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}
```
For convenience, when you use the microsyntax extends: "//", you can omit "root": false, because it is already implied that this configuration isn’t a root configuration:
packages/generate/biome.json
```
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{
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   "root": false,
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    "extends": "//",
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    "formatter": {
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    "enabled": false
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}
```
Now, let’s suppose we have a new package in packages/analytics that is maintained by a different team. This team follows entirely different coding standards, so they don’t want to inherit options from the root configuration. For them, they just need to omit "extends": "//" from the configuration file, and change the formatting options:
packages/analytics/biome.json
```
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{
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    "root": false,
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    "formatter": {
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        "lineWidth": 100,
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    }
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}
```
Now that everything is set up, you have a few options. You can run biome commands from the root of the project, or from the single packages. Biome will respect all settings!

As we saw above, the extends field allows you to split your configuration across multiple files. This way, you can share common settings across different projects or folders. The "extends": "//" syntax is a convenient shortcut when you want nested configurations to extend from the root configuration, but sometimes you may want to use an even more customised setup.

Apart from "//", the extends setting accept array values as well. In this case, every value in the array must be a path to another config to extend.

For example, here is how you might set up your configuration to extend a common.json configuration file:

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{
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  "extends": ["./common.json"]
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}

The entries defined in extends are resolved from the path where the biome.json file is defined. They are processed in the order they are listed, with settings in later files overriding earlier ones.

Files that you extend from cannot extend other files in turn.

Note that paths in a configuration file are always resolved from the folder in which the biome.json/biome.jsonc file resides. When using the extends field, this means that paths in a shared configuration are interpreted from the location of the configuration that is extending it, and not from the folder of the file being extended.

For example, let’s assume a project that contains two directories backend/ and frontend/, each having their own biome.json that both extend a common.json configuration in the root folder:

Directorybackend/
- Directorysrc/
  - …
- Directorytest/
  - …
- biome.json
Directoryfrontend/
- Directorysrc/
  - …
- Directorytest/
  - …
- biome.json
common.json

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{
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  "files": {
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    "includes": ["src/**/*.js", "test/**/*.js"],
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  },
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  "linter": {
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    "includes": ["**", "!test/**"]
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  }
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}

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{
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  "extends": ["../common.json"]
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}

When running Biome from the frontend/ folder, it will be configured to format and lint all JavaScript files in the frontend/src/ and frontend/test/ folders, and only format the files in the frontend/src/ folder. This works because the paths specified in common.json are interpreted from the frontend/ folder, because that’s where the biome.json file resides.
Assuming backend/biome.json looks the same as frontend/biome.json, it will have the same behaviour, except the paths will be interpreted from the backend/ folder.

Note that in this setup, both frontend/biome.json and backend/biome.json are considered root configurations. You won’t be able to run Biome from the root of the repository, unless you use the --config-path CLI option and point it at one of the configurations.

Exporting a Biome configuration from an NPM package

Biome is also able to resolve configuration files from the node_modules/ folder. So you can export your configuration file from a package, and import it in multiple projects.

In order to do so, the first thing to do is to set up your “shared” Biome configuration in a certain way. Let’s suppose that you want to share a configuration from a package called @org/shared-configs, using the specifier @org/shared-configs/biome. You have to create an exports entry in the package.json of this package:

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{
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  "name": "@org/shared-configs",
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  "type": "module",
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  "exports": {
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    "./biome": "./biome.json"
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  }
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}

Make sure that @org/shared-configs is correctly installed in your project, and update the biome.json file to look like the following snippet:

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{
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  "extends": ["@org/shared-configs/biome"]
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}

Biome will attempt to resolve your library @org/shared-configs/ from your working directory. The working directory is:

when using the CLI, the folder where you execute your scripts from. Usually it matches the location of your package.json file;
when using the LSP, the root folder of your project.