TypeScript Compiler Internals

The TypeScript compiler source is located under the src/compiler folder.

It is split into the follow key parts:

• Scanner (scanner.ts)

• Parser (parser.ts)

• Binder (binder.ts)

• Checker (checker.ts)

• Emitter (emitter.ts)

Each of these get their own unique files in the source. These parts will be explained later on in this chapter.

BYOTS

We have a project called Bring Your Own TypeScript (BYOTS) which makes it easier to play around with the compiler API e.g. by exposing internal APIs. You can use it to expose your local app's version of TypeScript globally.

Syntax vs. Semantics

Just because something is syntactically correct doesn't mean it is semantically correct. Consider the following piece of TypeScript code which although syntactically valid is semantically wrong

var foo: number = "not a number";

Semantic means "meaning" in English. This concept is useful to have in your head.

Processing Overview

The following is a quick review of how these key parts of the TypeScript compiler compose:

SourceCode ~~ scanner ~~> Token Stream

Token Stream ~~ parser ~~> AST

AST ~~ binder ~~> Symbols

Symbol is the primary building block of the TypeScript semantic system. As shown the symbols are created as a result of binding. Symbols connect declaration nodes in the AST to other declarations contributing to the same entity.

Symbols + AST are what is used by the checker to semantically validate the source code

AST + Symbols ~~ checker ~~> Type Validation

Finally When a JS output is requested:

AST + Checker ~~ emitter ~~> JS

There are a few additional files in the TypeScript compiler that provide utilities to many of these key portions which we cover next.

File: Utilities

core.ts : core utilities used by the TypeScript compiler. A few important ones:

• let objectAllocator: ObjectAllocator : is a variable defined as a singleton global. It provides the definitions for getNodeConstructor (Nodes are covered when we look at parser / AST), getSymbolConstructor (Symbols are covered in binder), getTypeConstructor (Types are covered in checker), getSignatureConstructor (Signatures are the index, call and construct signatures).

File: Key Data Structures

types.ts contains key data structures and interfaces uses throughout the compiler. Here is a sampling of a few key ones:

• SyntaxKind

  The AST node type is identified by the SyntaxKind enum.

• TypeChecker

  This is the interface provided by the TypeChecker.

• CompilerHost

  This is used by the Program to interact with the System.

• Node

  An AST node.

File: System

system.ts. All interaction of the TypeScript compiler with the operating system goes through a System interface. Both the interface and its implementations (WScript and Node) are defined in system.ts. You can think of it as the Operating Environment (OE).

Now that you have an overview of the major files, we can look at the concept of Program