Typescript Generics

I want to create a class who's signature is modified by function calls. For example;

const thing = new Thing(); // Thing<"">

const thing = new Thing(); // Thing<"">

const thing = new Thing(); // Thing<"foo" | "bar">
thing.add("foo"); // Thing<"foo" | "bar">
thing.add("bar"); // Thing<"foo" | "bar">

const thing = new Thing(); // Thing<"foo" | "bar">
thing.add("foo"); // Thing<"foo" | "bar">
thing.add("bar"); // Thing<"foo" | "bar">

I know that I can add to the generic by chaining .add().add() and having add's signature be add<T>(arg: T): Thing<ThingGeneric | T>, but I would like to be able to perform some kind of operation where if add is called with some parameter T, then thing always has been of a type such that it's first generic includes T. Is this even remotely possible?

3 Replies

Rägnar O'ock•2y ago

it is not possible because TS can't know when, where or even if the altering method is called. you will need to chain your methods, use a buffer const/var or cast the value to achieve what you want (I would advise against casting as you might run into jard to diagnose issues in the future

WillsterJohnson•2y ago

I thought that might be the case The disconnect between data and types in TS is becoming more of a pain as I do more complex things

Rägnar O'ock•2y ago

I feel you, I had the same issue with a data parser, and I came to the hard truth same as you xD

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Typescript Generics