This repository is just a bit of fun that explores pushing the advanced features of TypeScript's type system to the limits by using it to generate a type for the lyrics of The 12 Days of Christmas.
There is an associated TypeScript 4.1 blog post that covers some of the features that are used.
Note that we are not executing code or using data, we're only defining types, but the types are compiled so that only the lyrics is a valid value for the type.
It utilises advanced TypeScript features,
- Const Assertions (3.4+)
- Template Literal Types (4.1+)
- Conditional Types
- Mapped Types
- Variadic Tuple Types (4.0+)
- Recursive Conditionals (4.1+)
The motivation was really the inclusion of Recursive Conditional Types and Template Literal Types in TypeScript 4.1.
You will need to ensure you are using TypeScript 4.1 (currently RC)
so perform an npm install and set your IDE to use the workspace
version of TypeScript.
If you build, the code WILL give errors. This is just a bit of fun and pushes the compiler further than you would in normal production code so you will see lots of errors like,
Type instantiation is excessively deep and possibly infinite.Excessive stack depth comparing types ...
I could make it a bit more complex and count the recursive loops and bomb out after a certain number but haven't done it.
The easiest way to see the result is to open tests.ts in Visual
Studio Code and hold Ctrl while hovering over Solution.
I work for a software development company called Instil Software which also provides training. We train teams all over the world on all sorts of topics (not just TypeScript).
Check out our TypeScript course promo video and some of our courses,
Lot's of inspiration from ts-toolbelt.