Type Checker

Relevant source files

• src/compiler/binder.ts
• src/compiler/checker.ts
• src/compiler/commandLineParser.ts
• src/compiler/core.ts
• src/compiler/diagnosticMessages.json
• src/compiler/emitter.ts
• src/compiler/moduleNameResolver.ts
• src/compiler/moduleSpecifiers.ts
• src/compiler/parser.ts
• src/compiler/program.ts
• src/compiler/resolutionCache.ts
• src/compiler/scanner.ts
• src/compiler/sys.ts
• src/compiler/types.ts
• src/compiler/utilities.ts
• src/harness/client.ts
• src/harness/fourslashImpl.ts
• src/harness/fourslashInterfaceImpl.ts
• src/harness/harnessLanguageService.ts
• src/server/editorServices.ts
• src/server/project.ts
• src/server/protocol.ts
• src/server/scriptInfo.ts
• src/server/scriptVersionCache.ts
• src/server/session.ts
• src/server/utilities.ts
• src/services/codefixes/importFixes.ts
• src/services/completions.ts
• src/services/exportInfoMap.ts
• src/services/findAllReferences.ts
• src/services/goToDefinition.ts
• src/services/importTracker.ts
• src/services/services.ts
• src/services/types.ts
• src/services/utilities.ts
• src/testRunner/unittests/tsserver/exportMapCache.ts
• tests/baselines/reference/api/typescript.d.ts
• tests/baselines/reference/callChain.2.js
• tests/baselines/reference/callChain.3.js
• tests/baselines/reference/callChain.types
• tests/baselines/reference/callChainInference.types
• tests/baselines/reference/conditionalTypes1.errors.txt
• tests/baselines/reference/conditionalTypes1.js
• tests/baselines/reference/conditionalTypes1.symbols
• tests/baselines/reference/conditionalTypes1.types
• tests/baselines/reference/constructorImplementationWithDefaultValues2.errors.txt
• tests/baselines/reference/controlFlowOptionalChain.errors.txt
• tests/baselines/reference/controlFlowOptionalChain.js
• tests/baselines/reference/controlFlowOptionalChain.symbols
• tests/baselines/reference/controlFlowOptionalChain.types
• tests/baselines/reference/controlFlowOptionalChain2.js
• tests/baselines/reference/controlFlowOptionalChain2.symbols
• tests/baselines/reference/controlFlowOptionalChain2.types
• tests/baselines/reference/controlFlowSuperPropertyAccess.types
• tests/baselines/reference/declarationEmitAliasInlineing.js
• tests/baselines/reference/declarationEmitAliasInlineing.symbols
• tests/baselines/reference/declarationEmitAliasInlineing.types
• tests/baselines/reference/derivedGenericClassWithAny.errors.txt
• tests/baselines/reference/discriminantPropertyCheck.js
• tests/baselines/reference/discriminantPropertyCheck.symbols
• tests/baselines/reference/discriminantPropertyCheck.types
• tests/baselines/reference/elementAccessChain.2.js
• tests/baselines/reference/elementAccessChain.3.js
• tests/baselines/reference/genericCallWithFunctionTypedArguments.errors.txt
• tests/baselines/reference/genericCallWithGenericSignatureArguments2.errors.txt
• tests/baselines/reference/genericCallWithObjectTypeArgsAndInitializers.errors.txt
• tests/baselines/reference/genericCallbackInvokedInsideItsContainingFunction1.errors.txt
• tests/baselines/reference/genericClassWithFunctionTypedMemberArguments.errors.txt
• tests/baselines/reference/genericWithOpenTypeParameters1.errors.txt
• tests/baselines/reference/indexedAccessTypeConstraints.js
• tests/baselines/reference/indexedAccessTypeConstraints.symbols
• tests/baselines/reference/indexedAccessTypeConstraints.types
• tests/baselines/reference/inferTypes1.errors.txt
• tests/baselines/reference/inferTypes1.js
• tests/baselines/reference/inferTypes1.symbols
• tests/baselines/reference/inferTypes1.types
• tests/baselines/reference/mappedTypeNestedGenericInstantiation.js
• tests/baselines/reference/mappedTypeNestedGenericInstantiation.symbols
• tests/baselines/reference/mappedTypeNestedGenericInstantiation.types
• tests/baselines/reference/propertyAccessChain.2.js
• tests/baselines/reference/propertyAccessChain.3.js
• tests/baselines/reference/superMethodCall.types
• tests/baselines/reference/templateLiteralTypes4.types
• tests/baselines/reference/thisMethodCall.types
• tests/baselines/reference/tsserver/exportMapCache/invalidates-the-cache-when-a-file-is-opened-with-different-contents.js
• tests/baselines/reference/typeArgumentsOnFunctionsWithNoTypeParameters.errors.txt
• tests/baselines/reference/typeParameterLeak.js
• tests/baselines/reference/typeParameterLeak.symbols
• tests/baselines/reference/typeParameterLeak.types
• tests/baselines/reference/typeVariableTypeGuards.js
• tests/baselines/reference/typeVariableTypeGuards.symbols
• tests/baselines/reference/typeVariableTypeGuards.types
• tests/baselines/reference/unionOfClassCalls.types
• tests/baselines/reference/voidReturnIndexUnionInference.types
• tests/cases/compiler/declarationEmitAliasInlineing.ts
• tests/cases/compiler/discriminantPropertyCheck.ts
• tests/cases/compiler/indexedAccessTypeConstraints.ts
• tests/cases/compiler/mappedTypeNestedGenericInstantiation.ts
• tests/cases/compiler/typeParameterLeak.ts
• tests/cases/compiler/typeVariableTypeGuards.ts
• tests/cases/conformance/controlFlow/controlFlowOptionalChain.ts
• tests/cases/conformance/controlFlow/controlFlowOptionalChain2.ts
• tests/cases/fourslash/fourslash.ts
• tests/cases/fourslash/server/completionsOverridingMethodCrash2.ts

The type checker is the semantic analysis core of the TypeScript compiler. It consumes the AST produced by the parser and the symbol table built by the binder, then performs symbol resolution, type inference, type compatibility checking, and generates all semantic diagnostics. The type checker is implemented in src/compiler/checker.ts, which is by far the largest file in the repository (~70,000 lines).

For parser and AST details, see 2.2 For binder and symbol table construction, see 2.3 For how Program manages the checker lifecycle, see 2.6 For the public-facing TypeScript compiler API, see 7

---

Position in the Compilation Pipeline

The checker is created lazily by Program.getTypeChecker() via createTypeChecker in src/compiler/program.ts src/compiler/program.ts50 It wraps all bound source files from the program and performs semantic analysis on demand.

Compilation Pipeline — Code Entity Mapping

Sources: src/compiler/program.ts50 src/compiler/checker.ts1-1151 src/compiler/emitter.ts1-426

---

Entry Point: createTypeChecker

createTypeChecker(host: TypeCheckerHost): TypeChecker is the sole export from src/compiler/checker.ts that external consumers use src/compiler/checker.ts1157 It returns an object implementing:

• TypeChecker — the public interface defined in src/compiler/types.ts src/compiler/types.ts4012-4144 consumed by the language service and public API.
• EmitResolver — an internal interface consumed by the emitter and declaration emitter to answer questions about symbol visibility, constant values, and emit-relevant type information src/compiler/types.ts4152-4214

The factory closes over all internal state: relation maps, type and symbol caches, diagnostic arrays, and configuration derived from CompilerOptions.

Sources: src/compiler/checker.ts1-1151 src/compiler/program.ts50 src/compiler/types.ts4012-4214

---

Core Data Structures

The Type Interface and TypeFlags

Every type in the TypeScript type system is represented by the Type interface in src/compiler/types.ts src/compiler/types.ts3661-3666 The flags: TypeFlags bitmask field identifies the exact variety of each type instance src/compiler/types.ts3581-3659

Type Hierarchy via TypeFlags

Key TypeFlags groupings in src/compiler/types.ts:

Group	Members
Nullable	`Undefined
StringLike	`String
NumberLike	`Number
StructuredType	`Object
Instantiable	`TypeParameter
Narrowable	`Any

Sources: src/compiler/types.ts3581-3666 src/compiler/checker.ts459-1150

ObjectFlags

Object types carry a second bitmask, objectFlags: ObjectFlags, that further classifies them src/compiler/types.ts3692-3715 Key values:

Flag	Meaning
Reference	Instantiation of a generic type (e.g. Array<string>) src/compiler/types.ts3695
Tuple	Tuple type src/compiler/types.ts3696
Anonymous	Object literal type or function type src/compiler/types.ts3697
Mapped	Mapped type src/compiler/types.ts3698
ObjectLiteral	Fresh object literal type src/compiler/types.ts3701
EvolvingArray	Narrowing-evolved array type src/compiler/types.ts3702

Sources: src/compiler/types.ts3692-3715 src/compiler/checker.ts930-940

Symbol and SymbolFlags

Symbol objects are created by the binder and decorated by the checker src/compiler/types.ts3522-3532 The flags: SymbolFlags bitmask identifies what kind of entity a symbol represents src/compiler/types.ts3464-3518

Flag Category	Members
Value declarations	FunctionScopedVariable, BlockScopedVariable, Property, EnumMember, Function, Class, Enum, ValueModule, GetAccessor, SetAccessor, Method, Constructor src/compiler/types.ts3510
Type declarations	Interface, TypeAlias, TypeParameter, TypeLiteral src/compiler/types.ts3511
Alias	Alias (imports, re-exports) src/compiler/types.ts3489
Transient	Transient (checker-synthesized, e.g. mapped type members) src/compiler/types.ts3503

Sources: src/compiler/types.ts3464-3532 src/compiler/checker.ts256-262

---

Per-Node and Per-Symbol Caches

The checker avoids redundant computation by associating cached data with nodes and symbols:

• NodeLinks — attached to AST nodes via a node id. Stores: resolved type (resolvedType), resolved signature, and flags like NodeCheckFlags src/compiler/types.ts3944-3984
• SymbolLinks — attached to Symbol objects. Stores: declared type, resolved type, and more src/compiler/types.ts3914-3942

NodeCheckFlags encodes semantic properties discovered during checking, such as whether a super call is needed or whether a binding is captured across a closure boundary src/compiler/types.ts3986-4010

Sources: src/compiler/types.ts3914-4010 src/compiler/checker.ts916-920

---

Type Checking and Inference

Expression Checking: getTypeOfExpression and CheckMode

The central internal function for deriving a type from an expression node is getTypeOfExpression. It delegates to checkExpression, which dispatches based on the expression's SyntaxKind. The CheckMode const enum controls how the check proceeds src/compiler/checker.ts1157-1259:

CheckMode	Description
Normal	Standard type checking src/compiler/checker.ts1158
Contextual	Deriving a contextual type from a parent expression src/compiler/checker.ts1160
Inferential	Type inference from an argument against a parameter type src/compiler/checker.ts1161

Expression Type Resolution Flow

Sources: src/compiler/checker.ts1157-1259 src/compiler/types.ts453-456

Type Inference: InferenceContext and InferencePriority

When the checker resolves a generic call expression, it creates an InferenceContext src/compiler/types.ts3849-3860 InferencePriority ranks candidates src/compiler/types.ts3827-3847:

Priority	Meaning
NakedTypeVariable	Direct inference against a type parameter src/compiler/types.ts3828
ReturnType	From return type src/compiler/types.ts3838
AlwaysStrict	Strict inference mode src/compiler/types.ts3845

Sources: src/compiler/types.ts3827-3860 src/compiler/checker.ts453-456

---

Type Compatibility

Relation Maps

The checker maintains several objects that cache the result of comparing two types. These are the key relations:

Relation	Use
assignableRelation	Is type S assignable to type T? src/compiler/checker.ts467
strictSubtypeRelation	Is S a strict subtype of T? src/compiler/checker.ts469
comparableRelation	Is S comparable to T? src/compiler/checker.ts468
identityRelation	Are S and T identical? src/compiler/checker.ts466

Assignability Checking Flow

Sources: src/compiler/checker.ts466-469 src/compiler/types.ts1-20

---

Control Flow and Type Narrowing

The checker uses the flow graph built by the binder to compute narrowed types for variables at specific AST locations.

TypeFacts

The TypeFacts bitmask describes which type guards can narrow a given type src/compiler/checker.ts1229-1259

Group	Meaning
Truthy, Falsy	Truthiness guards src/compiler/checker.ts1255-1256
IsUndefined, IsNull	Type contains undefined/null src/compiler/checker.ts1258-1259

Sources: src/compiler/checker.ts1229-1259

---

Public TypeChecker Interface

The TypeChecker interface in src/compiler/types.ts is what the language service and external consumers use src/compiler/types.ts4012-4144

Type Query Methods

Method	Description
getTypeAtLocation(node)	Type of an expression or declaration node src/compiler/types.ts4013
getSymbolAtLocation(node)	Symbol referenced by a node src/compiler/types.ts4017
getResolvedSignature(call)	Resolved Signature for a call expression src/compiler/types.ts4043
getSemanticDiagnostics(sourceFile?)	All semantic diagnostics src/compiler/types.ts4139

Sources: src/compiler/types.ts4012-4144 tests/baselines/reference/api/typescript.d.ts1-200