https://github.com/stonecypher/is_a_ts

Typescript implementation of generic type guard, inherited from work by Ran Lottem
https://github.com/stonecypher/is_a_ts

Last synced: about 1 month ago
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Typescript implementation of generic type guard, inherited from work by Ran Lottem

• Host: GitHub
• URL: https://github.com/stonecypher/is_a_ts
• Owner: StoneCypher
• License: mit
• Created: 2021-07-24T17:38:37.000Z (over 3 years ago)
• Default Branch: main
• Last Pushed: 2021-07-30T15:04:18.000Z (over 3 years ago)
• Last Synced: 2024-11-25T04:48:40.548Z (about 2 months ago)
• Language: TypeScript
• Size: 280 KB
• Stars: 2
• Watchers: 4
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGELOG.md
  • License: LICENSE

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README

        
# is_a_ts

Typescript implementation of

[generic type guard, by Ran Lottem](https://dev.to/krumpet/generic-type-guard-in-typescript-258l)

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## 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](https://dev.to/krumpet/generic-type-guard-in-typescript-258l).

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."

```typescript

class SomeContainer {

  _storage: T[];

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

    const inst = new SomeContainer

    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](https://dev.to/krumpet/generic-type-guard-in-typescript-258l),

so here, we've productionalized it, and automated the test set.

It's now relatively simple.

```typescript

import { is_a } from 'is_a_ts';

class SomeContainer {

  _storage: T[];

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

    const inst = new SomeContainer

    source.forEach(item => {

      if (is_a(item, T)) {   // Okay then

        _storage.push(T);

      }

    });

  }

}

```

Nice and simple, the way code ought to be.