Architecture

This document covers some of the internals of Biome, and how they are used inside the project.

Scanner

Biome has a scanner that is responsible for crawling the file system to extract important metadata about projects. Specifically, there are three ways in which the scanner is used:

To discover nested biome.json/biome.jsonc files in monorepos.
To discover nested .gitignore files if the vcs.useIgnoreFile setting is enabled.
To index all JavaScript/TypeScript files in a project if any rules from the project domain are enabled.

Scanner targeting

If project rules are not enabled, the scanner automatically targets only the folders that are relevant for a given session.

This means that if you have a large monorepo, and you run biome check from inside the packages/foo/ folder, that folder will be “targeted”. This means the following folders get scanned for nested configuration files and/or nested ignore files:

The root folder of the repository.
The packages/ folder.
The packages/foo/ folder.
Any folders that exist under packages/foo/, except node_modules/ or those that are excluded by your configuration (see below).

Other folders that may be adjacent to either packages/ or packages/foo/ will be automatically skipped.

Similarly, if you run biome format packages/bar/src/index.ts from the root of the repository, the scanner will target the packages/bar/src/ folder.

If project rules are enabled, these optimisations don’t apply.

Configuring the scanner

The scanner respects the files.includes setting as well as any .gitignore files, but there is one major exception to be aware of:

If the project domain or one of its rules is enabled, dependencies of your included files are scanned as well. This means that .d.ts files and package.json manifests inside node_modules may still get indexed, provided there is a source file that references them. Similarly, generated files that are normally ignored, may still be indexed if you import them. This is done in order to extract type information from such files.

For the time being, if you really insist on preventing the scanner from accessing a given file or folder, you can use the files.experimentalScannerIgnores setting for that.

Parser and CST

The architecture of the parser is bumped by an internal fork of rowan, a library that implements the Green and Red tree pattern.

The CST (Concrete Syntax Tree) is a data structure very similar to an AST (Abstract Syntax Tree) that keeps track of all the information of a program, trivia included.

Trivia is represented by all that information that is important to a program to run:

spaces
tabs
comments

Trivia is attached to a node. A node can have leading trivia and trailing trivia. If you read code from left to right, leading trivia appears before a keyword, and trialing trivia appears after a keyword.

Leading trivia and trailing trivia are categorized as follows:

Every trivia up to the token/keyword (including line breaks) will be the leading trivia;
Everything until the next linebreak (but not including it) will be the trailing trivia;

Given the following JavaScript snippet, // comment 1 is a trailing trivia of the token ;, and // comment 2 is a leading trivia to the keyword const. Below is a minimized version of the CST represented by Biome:

1
const a = "foo"; // comment 1
2
// comment 2
3
const b = "bar";

1
0: JS_MODULE@0..55
2
    ...
3
      1: SEMICOLON@15..27 ";" [] [Whitespace(" "), Comments("// comment 1")]
4
    1: JS_VARIABLE_STATEMENT@27..55
5
        ...
6
        1: CONST_KW@27..45 "const" [Newline("\n"), Comments("// comment 2"), Newline("\n")] [Whitespace(" ")]
7
  3: EOF@55..55 "" [] []

The CST is never directly accessible by design; a developer can read its information using the Red tree, using a number of APIs that are autogenerated from the grammar of the language.

Resilient and recoverable parser

In order to construct a CST, a parser needs to be error-resilient and recoverable:

resilient: a parser that is able to resume parsing after encountering syntax errors that belong to the language;
recoverable: a parser that is able to understand where an error occurred and being able to resume the parsing by creating correct information;

The recoverable part of the parser is not a science, and no rules are set in stone. This means that depending on what the parser was parsing and where an error occurred, the parser might be able to recover itself in an expected way.

The parser also uses’ Bogus’ nodes to protect the consumers from consuming incorrect syntax. These nodes are used to decorate the broken code caused by a syntax error.

In the following example, the parentheses in the while are missing, although the parser can recover itself in a good manner and can represent the code with a decent CST. The parenthesis and condition of the loop are marked as missing, and the code block is correctly parsed:

1
while {}

1
JsModule {
2
  interpreter_token: missing (optional),
3
  directives: JsDirectiveList [],
4
  items: JsModuleItemList [
5
    JsWhileStatement {
6
      while_token: WHILE_KW@0..6 "while" [] [Whitespace(" ")],
7
      l_paren_token: missing (required),
8
      test: missing (required),
9
      r_paren_token: missing (required),
10
      body: JsBlockStatement {
11
        l_curly_token: L_CURLY@6..7 "{" [] [],
12
        statements: JsStatementList [],
13
        r_curly_token: R_CURLY@7..8 "}" [] [],
14
      },
15
    },
16
  ],
17
  eof_token: EOF@8..8 "" [] [],
18
}

This is an error emitted during parsing:

1
main.tsx:1:7 parse ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
2

3
  ✖ expected `(` but instead found `{`
4

5
  > 1 │ while {}
6
      │       ^
7

8
  ℹ Remove {

The same can’t be said for the following snippet. The parser can’t properly understand the syntax during the recovery phase, so it needs to rely on the bogus nodes to mark some syntax as erroneous. Notice the JsBogusStatement:

1
function}

1
JsModule {
2
  interpreter_token: missing (optional),
3
  directives: JsDirectiveList [],
4
  items: JsModuleItemList [
5
    TsDeclareFunctionDeclaration {
6
      async_token: missing (optional),
7
      function_token: FUNCTION_KW@0..8 "function" [] [],
8
      id: missing (required),
9
      type_parameters: missing (optional),
10
      parameters: missing (required),
11
      return_type_annotation: missing (optional),
12
      semicolon_token: missing (optional),
13
    },
14
    JsBogusStatement {
15
      items: [
16
        R_CURLY@8..9 "}" [] [],
17
      ],
18
    },
19
  ],
20
  eof_token: EOF@9..9 "" [] [],
21
}

This is the error we get from the parsing phase:

1
main.tsx:1:9 parse ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
2

3
  ✖ expected a name for the function in a function declaration, but found none
4

5
  > 1 │ function}
6
      │         ^

Formatter

Linter

Daemon

Biome uses a server-client architecture to run its tasks.

A daemon is a long-running server that Biome spawns in the background and uses to process requests from the editor and CLI.