Configuration

One of Ultracite’s best features is that it’s zero-config out of the box – you can just extend the ultracite preset and get going. However, every project is a bit different, and you might want to customize certain rules or adapt Ultracite for different frameworks. This section explains the default configuration and how to extend or override it.

Default configuration

When you include "extends": ["ultracite"] in your biome.json, you are pulling in Ultracite’s preset configuration. This preset includes a wide array of rules and configurations. Some notable aspects of the default config:

• TypeScript strictness: Ultracite enables TypeScript’s strict checks and lint rules requiring robust typing. For instance, it discourages use of any, requires handling of null/undefined, and prefers explicit types in certain situations to avoid ambiguity.
• ESLint "Recommended" equivalents: Common best-practice rules (no unused vars, no explicit eval, no proto pollution, etc.) are included.
• React and Accessibility: As mentioned, rules equivalent to React’s best practices and accessibility standards are on by default (if relevant to your files).
• Next.js and Node: Environment-specific quirks are handled. For example, global variables provided by Next.js or Node (like module, process) are recognized so they won’t be incorrectly flagged as undefined. The config knows about .next/ build folder to ignore it, etc.
• Formatting conventions: Ultracite’s formatter settings (largely inherited from Biome/Prettier defaults) include 2-space indentation, a max line width of 80 characters, semicolons at ends of statements, trailing commas where valid (e.g., in multi-line arrays/objects), single quotes for strings (except JSON, which uses double quotes), etc. These are chosen to match common conventions and ensure diff cleanliness.

All these defaults aim to enforce consistent style and prevent common errors without you having to manually configure each rule.

Overriding the configuration

Modifying biome.json

You can disable rules by modifying the biome.json file.

For example, to disable the noAutofocus rule, you can do the following:

{
  "$schema": "https://biomejs.dev/schemas/1.9.4/schema.json",
  "extends": ["ultracite"],
  "linter": {
    "rules": {
      "a11y": {
        "noAutofocus": "off"
      }
    }
  }
}

Inline Comments

You can also disable rules on a per-line basis by adding a comment to the end of the line:

// biome-ignore lint/security/noDangerouslySetInnerHtml: This is a valid use case.
<div dangerouslySetInnerHTML={{ ... }} />