TypeScript Compiler Notes
This repo is a corpus of notes from many engineers over time on different systems inside the TypeScript codebase. It is not meant as a “one true” source of authoritative documentation for the TypeScript compiler API, but as a way to ease contributions to the
If you’re already familiar with the TypeScript codebase and want to help out, we’re open to external folks sending PRs improving or adding areas!
If you are really short on time, here is a quick overview of the compilation process.
The process starts with preprocessing.
The preprocessor figures out what files should be included in the compilation by following references (
/// <reference path=... /> tags,
The parser then generates AST
These are just an abstract representation of the user input in a tree format.
SourceFile object represents an AST for a given file with some additional information like the file name and source text.
The binder then passes over the AST nodes and generates and binds
Symbol is created for each named entity.
There is a subtle distinction but several declaration nodes can name the same entity.
That means that sometimes different
Nodes will have the same
Symbol, and each
Symbol keeps track of its declaration
For example, a
class and a
namespace with the same name can merge and will have the same
The binder also handles scopes and makes sure that each
Symbol is created in the correct enclosing scope.
SourceFile (along with its
Symbols) is done through calling the
Symbols represent named entities as seen within a single file, but several declarations can merge multiple files, so the next step is to build a global view of all files in the compilation by building a
Program is a collection of
SourceFiles and a set of
Program is created by calling the
Program instance a
TypeChecker can be created.
TypeChecker is the core of the TypeScript type system.
It is the part responsible for figuring out relationships between
Symbols from different files, assigning
Symbols, and generating any semantic
Diagnostics (i.e. errors).
The first thing a
TypeChecker will do is to consolidate all the
Symbols from different
SourceFiles into a single view, and build a single Symbol Table by “merging” any common
namespaces spanning multiple files).
After initializing the original state, the
TypeChecker is ready to answer any questions about the program.
Such “questions” might be:
- What is the
- What is the
Symbols are visible in this portion of the AST?
- What are the available
Signatures for a function declaration?
- What errors should be reported for a file?
TypeChecker computes everything lazily; it only “resolves” the necessary information to answer a question.
The checker will only examine
Types that contribute to the question at hand and will not attempt to examine additional entities.
Emitter can also be created from a given
Emitter is responsible for generating the desired output for a given
SourceFile; this includes
From there, you can start in the First Steps to Contributing to the TypeScript Repo consult the Glossary or dive directly into the
One of the best places to ask questions is in the ‘compiler-internals-and-api’ channel of the TypeScript Community Discord.
Related TypeScript Info
- Learn how TypeScript works by reading the mini-TypeScript implementation
- Basarat’s guide to the Compiler Internals
Compilers in General
- Recommended link for learning how compilers work: https://c9x.me/compile/bib/
If you learn better by seeing how big features are added to TypeScript, here are a few big well-scoped Pull Requests:
- Unions – microsoft/TypeScript#824
- Type Aliases – microsoft/TypeScript#957
- Async Functions – microsoft/TypeScript#3078
- TSX – microsoft/TypeScript#3564
- Intersection Types – microsoft/TypeScript#3622
- String Literal Types – microsoft/TypeScript#5185
- JS in TS – microsoft/TypeScript#5266
- Using JSDoc to extract types – microsoft/TypeScript#6024
- Nullable types – microsoft/TypeScript#7140
- Control Flow Analysis – microsoft/TypeScript#8010
- Mapped Types – microsoft/TypeScript#12114
- Rest Types – microsoft/TypeScript#13470
- Strict Functions – microsoft/TypeScript#18654
- Unknown – microsoft/TypeScript#24439
- Optional Chaining – microsoft/TypeScript#33294
- Node ESM Support – microsoft/TypeScript#45884
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