is_a_ts

Typescript implementation of generic type guard, by Ran Lottem

What is this?

The mechanism that most Typescript developers use to distinguish types, instanceof, doesn't work at runtime - only compiletime. Making a runtime version is a complicated affair.

This library exists to centralize and test an function called is_a (aka typeGuard) to solve this problem.

... What is this?

Consider implementing .from on a custom container. Don't worry about the long signature; it's just an example.

To implement .from, your container has an internal typed Array, and so in implementing .from, you'd want to iterate over the input, to make sure all of the contents fit storage. Initially you might think "well, source is typed, should be fine."

class SomeContainer<T> {

  _storage: T[];

  static from<T>(source: Iterable<T> | ArrayLike<T>): T[] {

    const inst = new SomeContainer<T>

    source.forEach(item => {
      if (item instanceof T) {   // HA!
        _storage.push(T);
      }
    });

  }

}

See the place where The Joker is laughing at you? That's because instanceof is a compile time thing, and what you're doing requires runtime type inference.

Turns out there isn't a particularly good way to do this in TS. But I found one on some developer's blog, so here, we've productionalized it, and automated the test set.

It's now relatively simple.

import { is_a } from 'is_a_ts';

class SomeContainer<T> {

  _storage: T[];

  static from<T>(source: Iterable<T> | ArrayLike<T>): T[] {

    const inst = new SomeContainer<T>

    source.forEach(item => {
      if (is_a(item, T)) {   // Okay then
        _storage.push(T);
      }
    });

  }

}

Nice and simple, the way code ought to be.