type-inference

name	type-inference
description	Guide for working with Biome's module graph and type inference system. Use when implementing type-aware lint rules, understanding type resolution, working on the module graph infrastructure, or implementing type inference for new features.
compatibility	Designed for coding agents working on the Biome codebase (github.com/biomejs/biome).

Purpose

Use this skill when working with Biome's type inference system and module graph. Covers type references, resolution phases, and the architecture designed for IDE performance.

Prerequisites

1. Read crates/biome_js_type_info/CONTRIBUTING.md for architecture details
2. Understand Biome's focus on IDE support and instant updates
3. Familiarity with TypeScript type system concepts

Key Concepts

Module Graph Constraint

Critical rule: No module may copy or clone data from another module, not even behind Arc.

Why: Any module can be updated at any time (IDE file changes). Copying data would create stale references that are hard to invalidate.

Solution: Use TypeReference instead of direct type references.

Type Data Structure

Types are stored in TypeData enum with many variants:

// Simplified — see crates/biome_js_type_info/src/type_data.rs for the full enum
enum TypeData {
    Unknown,                            // Inference not implemented
    Global,                             // Global type reference
    BigInt, Boolean, Null, Number,      // Primitive types
    String, Symbol, Undefined,
    Function(Box<Function>),            // Function with parameters
    Object(Box<Object>),                // Object with properties
    Class(Box<Class>),                  // Class definition
    Interface(Box<Interface>),          // Interface definition
    Union(Box<Union>),                  // Union type (A | B)
    Intersection(Box<Intersection>),    // Intersection type (A & B)
    Tuple(Box<Tuple>),                  // Tuple type
    Literal(Box<Literal>),              // Literal type ("foo", 42)
    Reference(TypeReference),           // Reference to another type
    TypeofExpression(Box<TypeofExpression>), // typeof an expression
    // ... plus Conditional, Generic, TypeOperator, InstanceOf,
    //     keyword variants (AnyKeyword, NeverKeyword, VoidKeyword, etc.)
}

Type References

Instead of direct type references, use TypeReference:

enum TypeReference {
    Qualifier(Box<TypeReferenceQualifier>),  // Name-based reference
    Resolved(ResolvedTypeId),                 // Resolved to type ID
    Import(Box<TypeImportQualifier>),         // Import reference
}

Note: There is no Unknown variant. Unknown types are represented as TypeReference::Resolved(GLOBAL_UNKNOWN_ID). Use TypeReference::unknown() to create one.

Type Resolution Phases

1. Local Inference

What: Derives types from expressions without surrounding context.

Example: For a + b, creates:

TypeData::TypeofExpression(TypeofExpression::Addition {
    left: TypeReference::from(TypeReferenceQualifier::from_name("a")),
    right: TypeReference::from(TypeReferenceQualifier::from_name("b"))
})

Where: Implemented in local_inference.rs

Output: Types with unresolved TypeReference::Qualifier references

2. Module-Level ("Thin") Inference

What: Resolves references within a single module's scope.

Process:

1. Takes results from local inference
2. Looks up qualifiers in local scopes
3. Converts to TypeReference::Resolved if found locally
4. Converts to TypeReference::Import if from import statement
5. Falls back to globals (like Array, Promise)
6. Uses TypeReference::Unknown if nothing found

Where: Implemented in js_module_info/collector.rs

Output: Types with resolved local references, import markers, or unknown

3. Full Inference

What: Resolves import references across module boundaries.

Process:

1. Has access to entire module graph
2. Resolves TypeReference::Import by following imports
3. Converts to TypeReference::Resolved after following imports

Where: Implemented in js_module_info/module_resolver.rs

Limitation: Results cannot be cached (would become stale on file changes)

Working with Type Resolvers

Available Resolvers

// 1. For tests
HardcodedSymbolResolver

// 2. For globals (Array, Promise, etc.)
GlobalsResolver

// 3. For thin inference (single module)
JsModuleInfoCollector

// 4. For full inference (across modules)
ModuleResolver

Using a Resolver

use biome_js_type_info::{TypeResolver, ResolvedTypeData};

fn analyze_type(resolver: &impl TypeResolver, type_ref: TypeReference) {
    // Resolve the reference
    let resolved_data: ResolvedTypeData = resolver.resolve_type(type_ref);

    // Get raw data for pattern matching
    match resolved_data.as_raw_data() {
        TypeData::String => { /* handle string */ },
        TypeData::Number => { /* handle number */ },
        TypeData::Function(func) => { /* handle function */ },
        _ => { /* handle others */ }
    }

    // Resolve nested references
    if let TypeData::Reference(inner_ref) = resolved_data.as_raw_data() {
        let inner_data = resolver.resolve_type(*inner_ref);
        // Process inner type
    }
}

Type Flattening

What: Converts complex type expressions to concrete types.

Example: After resolving a + b:

• If both are TypeData::Number → Flatten to TypeData::Number
• Otherwise → Usually flatten to TypeData::String

Where: Implemented in flattening.rs

Common Workflows

Implement Type-Aware Lint Rule

use biome_analyze::Semantic;
use biome_js_type_info::{TypeResolver, TypeData};

impl Rule for MyTypeRule {
    type Query = Semantic<JsCallExpression>;

    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
        let node = ctx.query();
        let model = ctx.model();

        // Get type resolver from model
        let resolver = model.type_resolver();

        // Get type of expression
        let expr_type = node.callee().ok()?.infer_type(resolver);

        // Check the type
        match expr_type.as_raw_data() {
            TypeData::Function(_) => { /* valid */ },
            TypeData::Unknown => { /* might be valid, can't tell */ },
            _ => { return Some(()); /* not callable */ }
        }

        None
    }
}

Navigate Type References

fn is_string_type(resolver: &impl TypeResolver, type_ref: TypeReference) -> bool {
    let resolved = resolver.resolve_type(type_ref);

    // Follow references
    let data = match resolved.as_raw_data() {
        TypeData::Reference(ref_to) => resolver.resolve_type(*ref_to),
        _other => resolved,
    };

    // Check the resolved type
    matches!(data.as_raw_data(), TypeData::String)
}

Work with Function Types

fn analyze_function(resolver: &impl TypeResolver, type_ref: TypeReference) {
    let resolved = resolver.resolve_type(type_ref);

    if let TypeData::Function(func_type) = resolved.as_raw_data() {
        // Access parameters
        for param in func_type.parameters() {
            let param_type = resolver.resolve_type(param.type_ref());
            // Analyze parameter type
        }

        // Access return type
        let return_type = resolver.resolve_type(func_type.return_type());
    }
}

Architecture Principles

Why Type References?

Advantages:

1. No stale data: Module updates don't leave old types in memory
2. Better performance: Types stored in vectors (data locality)
3. Easier debugging: Can inspect all types in vector
4. Simpler algorithms: Process vectors instead of traversing graphs

Trade-off: Must explicitly resolve references (not automatic like Arc)

ResolvedTypeId Structure

struct ResolvedTypeId(ResolverId, TypeId)

• TypeId (u32): Index into a type vector
• ResolverId (u32): Identifies which vector to use
• Total: 64 bits (compact representation)

ResolvedTypeData

Always work with ResolvedTypeData from resolver, not raw &TypeData:

// Good - tracks resolver context
let resolved_data: ResolvedTypeData = resolver.resolve_type(type_ref);

// Be careful - loses resolver context
let raw_data: &TypeData = resolved_data.as_raw_data();
// Can't resolve nested TypeReferences without ResolverId!

Tips

• Unknown types: TypeData::Unknown means inference not implemented, treat as "could be anything"
• Follow references: Always follow TypeData::Reference to get actual type
• Resolver context: Keep ResolvedTypeData when possible, don't extract raw TypeData early
• Performance: Type vectors are fast - iterate directly instead of recursive traversal
• IDE focus: All design decisions prioritize instant IDE updates over CLI performance
• No caching: Full inference results can't be cached (would become stale)
• Globals: Currently hardcoded, eventually should use TypeScript's .d.ts files

Common Patterns

// Pattern 1: Resolve and flatten
let type_ref = expr.infer_type(resolver);
let flattened = type_ref.flatten(resolver);

// Pattern 2: Check if type matches
fn is_string_type(resolver: &impl TypeResolver, type_ref: TypeReference) -> bool {
    let resolved = resolver.resolve_type(type_ref);
    matches!(resolved.as_raw_data(), TypeData::String)
}

// Pattern 3: Handle unknown gracefully
match resolved.as_raw_data() {
    TypeData::Unknown | TypeData::UnknownKeyword => {
        // Can't verify, assume valid
        return None;
    }
    TypeData::String => { /* handle */ }
    _ => { /* handle */ }
}

References

• Architecture guide: crates/biome_js_type_info/CONTRIBUTING.md
• Module graph: crates/biome_module_graph/
• Type resolver trait: crates/biome_js_type_info/src/resolver.rs
• Flattening: crates/biome_js_type_info/src/flattening.rs