Theo's Typesafe Cult•16mo ago

Spreading Rest Arguments

I'm attempting to write a function which eventually calls another function with the initially provided arguments. I'm having trouble getting it to work, so any help would be much appreciated! Playground: https://www.typescriptlang.org/play?#code/PTAEDECcHsFtQC4AsCWkAmBaADgQ0ggJ6gA2KARpPoQFwBQAxtAHYDOCosh4ArswwhQtWoALygA3nVCgAZmna9+glgAp8AcwCMNUO0gpmGgDShNAJl3MescgFNIASknSZiJDADuoZne8BRSBhIVQByADloDhRYbBI7WDtmBDt0UMdXAF9jV1Y7JmZ0JQEhZnVIbV1yFA1DBFMLKxt7SFAAHx8eEhIGioBmKuhoeNxmZyk3dy8fP1BA4LDI6Nj4xOTU9Ky6TLpGElxWEQAVO3YXGWwecjIGUDsAD3yeFOKVZgAeI4BpO2IHlMKIgA1r9oLJEIRsHYwZxuHwSsIAHyqEG0UDfX6mAB0OM0rF0AAV8LhEilIKx3kQoTCuK9SqwANoYwgAXUR41cMhAoAAItA9HA7HoEDxZOCcVjOaBIHYRZBmLC6cIGaiWaoJXiMjIdpkgA

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9 Replies

andersgee•16mo ago

looked at this and your issue is essentially that the "...args" is a union of tuples, but must be a single tuple you must infer which one of the tuples it is somehow, probably with a switch statement or some typescript wizardry 😄

Christoph•16mo ago

The thing is, that the code, the auto-completion and type-hinting when using the function all works. It's really just a type error that I don't understand. Parameters<typeof myFunctions[TKey]> only returns the array of parameters needed for the required function. (e.g. type ParamsOfFirstFunction = Parameters<typeof myFunctions['firstFunction']> only returns one array/tuple).

andersgee•16mo ago

personally I would rewrite it so that your method takes 1 instead of 2, like this

// From third-party library:
const myFunctions = {
  firstFunction(arg1: string, arg2: number) {
    throw new Error("Not implemented");
  },
  secondFunction(arg1: bigint, arg2: number | null, arg3: boolean) {
    throw new Error("Not implemented");
  },
};

type Action =
  | {
      key: "firstFunction";
      args: Parameters<typeof myFunctions["firstFunction"]>;
    }
  | {
      key: "secondFunction";
      args: Parameters<typeof myFunctions["secondFunction"]>;
    };

export class Test {
  public executeFunction(action: Action) {
    switch (action.key) {
      case "firstFunction":
        myFunctions.firstFunction(...action.args);
        break;
      case "secondFunction":
        myFunctions.secondFunction(...action.args);
        break;
    }
  }
}

// From third-party library:
const myFunctions = {
  firstFunction(arg1: string, arg2: number) {
    throw new Error("Not implemented");
  },
  secondFunction(arg1: bigint, arg2: number | null, arg3: boolean) {
    throw new Error("Not implemented");
  },
};

type Action =
  | {
      key: "firstFunction";
      args: Parameters<typeof myFunctions["firstFunction"]>;
    }
  | {
      key: "secondFunction";
      args: Parameters<typeof myFunctions["secondFunction"]>;
    };

export class Test {
  public executeFunction(action: Action) {
    switch (action.key) {
      case "firstFunction":
        myFunctions.firstFunction(...action.args);
        break;
      case "secondFunction":
        myFunctions.secondFunction(...action.args);
        break;
    }
  }
}

Christoph•16mo ago

Hmm, let me see if that works when I make it generic.

Christoph•16mo ago

https://www.typescriptlang.org/play?#code/PTAEDECcHsFtQC4AsCWkAmBaADgQ0ggJ6gA2KARpPoQFwBQAxtAHYDOCosh4ArswwhQtWoALygA3nVCgAZmna9+glgAp8AcwCMNUO0gpmGgDShNAJl3MescgFNIASknSZiJDADuoZne8BRSBhIVQByADloDhRYbBI7WDtmBDt0UMdXAF9jV1Y7JmZ0JQEhZnVIbV1yFA1DBFMLKxt7SFAAHx8eEhIGioBmKuhoeNxmZyk3dy8fP1BA4LDI6Nj4xOTU9Ky6TLo6Imw7UABBEpYxFxkAbQBpUENQAGs7QmhZREID185uPlO2AF1dBM3E9aKBrjlJppWLoAAr4XCJFKQVgAHn2di+XGKKjYN3+AD4tplLqCvhisT9lKVWP9dowSLhWCIACp2dgXUDYHjkMgMUB2AAe+R4KRxpXUf10J1x41cMhAoAAItA9HBDuweLI3gA6PXy0CQOwIHiQZjfcXCS64P460H-VR6nU23HOiqsDIyHY7RjCDgpDniXzeNnsVQZAMIHVCkVi3641QSR7PXQAInkKIQluYqd6GhhoEuoU80EgD1YoVMWn+oEyEfZUZjDFFdmzieTYPTCiz8dKubM7t0RdYSGgnhEGNAqGSlcQkB4dhrdaAA

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Christoph•16mo ago

Unfortunately, the exact same error. And manually switching through all possible actions is not an option. The functions can change from time to time. Does anyone else have an idea?

awexis•16mo ago

https://tsplay.dev/m0YeDW does this do what you want?

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awexis•16mo ago

@Christoph

Christoph•16mo ago

That's not quite right. When you call the function now, arg1 would be the array of arguments and arg2 would be undefined.

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Spreading Rest Arguments