TypeScript & runtime type-checking, simple solution in 2020 [closed]

Question

As we all know, TypeScript type-checks only at compile-time.

There are couple of existing approaches to add runtime checks, such as io-ts, but I'm left to wonder if there is a simpler way.

For example a Babel plugin that would transpile this:

type Todo = {
  userId: number;
  id: number;
  title: string;
  completed: boolean;
}

const resp = await fetch("https://jsonplaceholder.typicode.com/todos/1");

const data = await resp.json();

assert(data typeof Todo);

to that:

const __TodoType = {
  userId: Number;
  id: Number;
  title: String;
  completed: Boolean;
};
const __isTodoType = obj => (
  obj &&
  obj.constructor === Object &&
  Object.keys(obj).length === Object.keys(__TodoType).length &&
  Object.entries(obj)
    .every(([prop, val]) =>
      __TodoType[prop] && val &&
      __TodoType[prop] === val.constructor)
);

const resp = await fetch("https://jsonplaceholder.typicode.com/todos/1");

const data = await resp.json();

assert(__isTodoType(data));

This would be a super simple solution and would cover many (if not most) uses cases. AFAICT, this would be enough for asserting serialization / data fetching.

Has someone managed to build such Babel plugin?

Edit - I know the existing libraries such as io-ts but I'm looking for something much simpler. The Babel Plugin I'm showcasing is vastly simpler (from the perspective of the plugin user) than anything else I've seen so far. I'm not sure why this hasn't been done before.

Answer 1

I was frustrated by this as well and ended up writing my own type guard generator called ‛ts-type-checked‛ available as an NPM module. It is a TypeScript transform that generates type guards based on your types.