type-exp

Type Expression

Currently in development

Concept

Explain Type using TypeScript, Treat type as data. Thanks Type-Level Programming by Gabriel Vergnaud, to make to exellent guide to understand clear in Type system of TypeScript.

here is an example, when we try to understand the type declaration for example Pick Utility Type, it quite hard to understand.

type MyPick<Obj, Keys extends keyof Obj> = {
    [Key in Keys]: Obj[Key];
};

This project try to describe the type by using TypeScript, and in the future, it should convert to the TypeScript type as well.

I convert it into the TypeScript function:

import { e } from 'type-exp';

function MyPick<T, K>(Obj: any, Keys: e.keyof<K>) {
    return e.map(
      Keys,
      (Key) => Key,
      (Key) => Obj[Key],
    );
  }

const todo = {
    title: e.string(),
    description: e.string(),
    completed: e.boolean(),
};

// Equivalent to =>
type TodoPreview = MyPick<Todo, "title" | "completed">;
console.log(MyPick(todo, e.union('title', 'description'));

// The result should be
// { title: 'string', description: 'string' }

Thanks

Template from https://github.com/jsynowiec/node-typescript-boilerplate