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 microsoft/TypeScript repo.
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 completely new to the TypeScript codebase, this YouTube video covers all of the major systems involved in converting files to JavaScript, and type-checking types which will provide a high level guide to what you may be interested in focusing on:
From there, you can start in the First Steps to Contributing to the TypeScript Repo consult the Glossary or dive directly into the ./codebase/ or ./systems/ folders.
One of the best places to ask questions is in the 'compiler-internals-and-api' channel of the TypeScript Community Discord.
- Learn how TypeScript works by reading the mini-TypeScript implementation
- Basarat's guide to the Compiler Internals
- 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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