The TypeScript compiler source is located under the
It is split into the follow key parts:
- Scanner (
- Parser (
- Binder (
- Checker (
- Emitter (
Each of these get their own unique files in the source. These parts will be explained later on in this chapter.
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.
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.
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
getSymbolConstructor(Symbols are covered in
getTypeConstructor(Types are covered in
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:
SyntaxKindThe AST node type is identified by the
TypeCheckerThis is the interface provided by the TypeChecker.
CompilerHostThis is used by the
Programto interact with the
NodeAn AST node.
system.ts. All interaction of the TypeScript compiler with the operating system goes through a
System interface. Both the interface and its implementations (
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