Linting and formatting

In an ideal world, your code is always clean, consistent, and free of pesky errors. That’s the promise of Deno’s built-in linting and formatting tools. By integrating these features directly into the runtime, Deno eliminates the need for external dependencies and complex configurations in your projects. These inbuilt tools are fast and performant, not only saving time but also ensuring that every line of code adheres to best practices.

With deno fmt and deno lint, you can focus on writing great code, knowing that Deno has your back. It’s like having a vigilant assistant who keeps your codebase in top shape, allowing you to concentrate on what truly matters: building amazing applications.

Linting

Explore all the lint rules

Linting is the process of analyzing your code for potential errors, bugs, and stylistic issues. Deno’s built-in linter, deno lint, supports recommended set of rules from ESLint to provide comprehensive feedback on your code. This includes identifying syntax errors, enforcing coding conventions, and highlighting potential issues that could lead to bugs.

To run the linter, use the following command in your terminal:

deno lint

By default, deno lint analyzes all TypeScript and JavaScript files in the current directory and its subdirectories. If you want to lint specific files or directories, you can pass them as arguments to the command. For example:

deno lint src/

This command will lint all files in the src/ directory.

The linter can be configured in a deno.json file. You can specify custom rules, plugins, and settings to tailor the linting process to your needs.

Linting rules

You can view and search the list of available rules and their usage on the List of rules documentation page.

Formatting

Formatting is the process of automatically adjusting the layout of your code to adhere to a consistent style. Deno’s built-in formatter, deno fmt, uses the powerful dprint engine to ensure that your code is always clean, readable, and consistent.

To format your code, simply execute the following command in your terminal:

deno fmt

By default, deno fmt formats all TypeScript and JavaScript files in the current directory and its subdirectories. If you want to format specific files or directories, you can pass them as arguments to the command. For example:

deno fmt src/

This command will format all files in the src/ directory.

Checking your formatting

The deno fmt --check command is used to verify if your code is properly formatted according to Deno’s default formatting rules. Instead of modifying the files, it checks them and reports any formatting issues. This is particularly useful for integrating into continuous integration (CI) pipelines or pre-commit hooks to ensure code consistency across your project.

If there are formatting issues, deno fmt --check will list the files that need formatting. If all files are correctly formatted, it will simply exit without any output.

Integration in CI

You can add deno fmt --check to your CI pipeline to automatically check for formatting issues. For example, in a GitHub Actions workflow:

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - uses: denoland/setup-deno@v2
        with:
          deno-version: v2.x
      - run: deno fmt --check

This ensures that any code changes adhere to the project’s formatting standards before being merged.

Available options

Rule	Description	Default	possible values
indent-width	Define indentation width	2	number
line-width	Define maximum line width	80	number
no-semicolons	Don't use semicolons except where necessary	false	true, false
prose-wrap	Define how prose should be wrapped	always	always, never, preserve
single-quote	Use single quotes	false	true, false
unstable-component	Enable formatting Svelte, Vue, Astro and Angular files
unstable-sql	Enable formatting SQL files
use-tabs	Use tabs instead of spaces for indentation	false	true, false

The formatter can be configured in a deno.json file. You can specify custom settings to tailor the formatting process to your needs.