Angular Compiler CLI (ngtsc) Architecture

Overview

The packages/compiler-cli package contains the Angular Compiler (Ivy), often referred to as ngtsc. It is a wrapper around the TypeScript compiler (tsc) that extends it with Angular-specific capabilities.

The core goal of ngtsc is to compile Angular decorators (like @Component, @Directive, @Pipe) into static properties on the class (Ivy instructions, e.g., static ɵcmp = ...). It also performs template type checking and ahead-of-time (AOT) compilation.

Mental Model

The compiler is designed as a lazy, incremental, and partial compilation pipeline.

1. Wrapper Pattern: NgtscProgram wraps the standard ts.Program. It intercepts calls to act as a drop-in replacement for standard tooling.
2. Traits System: Every class with an Angular decorator is considered a "Trait". The compiler manages the state of these traits through a state machine:
   • Pending: Detected but not processed.
   • Analyzed: Metadata extracted, template parsed (but dependencies not yet linked).
   • Resolved: Dependencies (directives/pipes in template) resolved, import cycles handled.
   • Skipped: Not an Angular class.
3. Lazy Analysis: Analysis only happens when necessary (e.g., when diagnostics are requested or emit is prepared).
4. Output AST: The compiler generates an intermediate "Output AST" (o.Expression) for the generated code, which is then translated into TypeScript AST nodes during the emit phase.

Key Subsystems

1. Core Orchestration (ngtsc/core)

• NgtscProgram: The public API implementing api.Program. It manages the ts.Program and the NgCompiler.
• NgCompiler: The brain of the compiler. It orchestrates the compilation phases (Analysis, Resolution, Type Checking, Emit). It holds the TraitCompiler.

2. Trait Compilation (ngtsc/transform)

• TraitCompiler: Manages the lifecycle of "Traits". It iterates over source files, identifies decorated classes, and delegates to the appropriate DecoratorHandler.
• Trait: A state container for a class, holding its handler, analysis results, and resolution results.

3. Decorator Handlers (ngtsc/annotations)

• DecoratorHandler: An interface for handling specific decorators.
• ComponentDecoratorHandler: The most complex handler. It:
  • Extracts metadata (selector, inputs, outputs).
  • Parses the template.
  • Resolves used directives and pipes (R3TargetBinder).
  • Generates the ɵcmp instruction.
• DirectiveDecoratorHandler, PipeDecoratorHandler, NgModuleDecoratorHandler: Handle their respective decorators.

4. Template Type Checking (ngtsc/typecheck)

• TemplateTypeChecker: Generates "Type Check Blocks" (TCBs). A TCB is a block of TypeScript code that represents the template's logic in a way tsc can understand and check for errors.
• TypeCheckBlock: The actual generated code that validates bindings, events, and structural directives.

5. Metadata & Scope (ngtsc/metadata, ngtsc/scope)

• MetadataReader: Reads Angular metadata from source files (using LocalMetadataRegistry) and .d.ts files (using DtsMetadataReader).
• ScopeRegistry: Determines the "compilation scope" of a component (which directives/pipes are available to it), handling NgModule transitive exports and Standalone Component imports.

6. Emit & Transformation (ngtsc/transform)

• ivyTransformFactory: A TypeScript transformer factory.
• IvyCompilationVisitor: Visits classes, triggers compilation via TraitCompiler, and collects the Output AST.
• IvyTransformationVisitor: Translates the Output AST into TypeScript AST, injects the static ɵ... fields, and removes the original decorators.

Compilation Phases

1. Construction: NgtscProgram creates NgCompiler, which sets up all registries and the TraitCompiler.
2. Analysis (analyzeSync):
   • The TraitCompiler scans files.
   • DecoratorHandlers extract metadata and parse templates.
   • No cross-file resolution happens here (allowing for parallelism and caching).
3. Resolution (resolve):
   • TraitCompiler resolves traits.
   • Components link their templates to specific Directives and Pipes (found via ScopeRegistry).
   • Import cycles are detected and handled (e.g., via "remote scoping").
4. Type Checking:
   • TemplateTypeChecker creates TCBs for all components.
   • TypeScript diagnostics are retrieved for these TCBs.
5. Emit (prepareEmit):
   • ivyTransformFactory is created.
   • TS emit is called.
   • The transformers run, injecting the compiled Ivy instructions into the JS/DTS output.

Important File Locations

• packages/compiler-cli/src/ngtsc/program.ts: Entry point (NgtscProgram).
• packages/compiler-cli/src/ngtsc/core/src/compiler.ts: Core logic (NgCompiler).
• packages/compiler-cli/src/ngtsc/transform/src/trait.ts: Trait state machine.
• packages/compiler-cli/src/ngtsc/annotations/component/src/handler.ts: Component compilation logic.
• packages/compiler-cli/src/ngtsc/typecheck/src/template_type_checker.ts: Type checking logic.
• packages/compiler-cli/src/ngtsc/transform/src/transform.ts: AST transformation logic.