propagate the amalgamation of tuples as an argument

I'm working with a function that returns a union type of tuples. I need to pass this return value to another function that can accept all its forms using the spread operator ....

type TupleUnion = readonly [number, number] | readonly [number, number, string]
function getTuple() : TupleUnion 
function calc(a, b, c?)
const tuple = getTuple();
calc(...tuple)

However, when I try to do this, the Typescript compiler gives me an error:

A spread argument must either have a tuple type or be passed to a rest parameter.ts(2556)

I don't want to redefine the calc function. Is there a way for the getTuple function to return a union type that supports the spread operator?

typescript

Answer №1

Utilize the variadic parameter in the calc() function and pass a tuple into it.

type TupleUnion = readonly [number, number] | readonly [number, number, string];
function getTuple(): TupleUnion
function calc(...a: TupleUnion)
const tuple = getTuple();
calc(...tuple);

Answer №2

The root cause of this issue lies in the fact that the calc function is bound by fixed parameters, whereas a tuple is mutable and can change.

To resolve this issue, one approach is to define a type with tuple during its declaration (as indicated by the error message):

const tuple: TupleUnion = getTuple();

An alternative solution would be to incorporate the type into the calc function and eliminate the fixed parameters:

function calc(...args: TupleUnion)

By adopting this method, you can utilize array syntax to access the elements within args similar to how you would interact with a tuple:

args[0]

A third option involves coercing tuple as a constant, although this workaround should only be considered if other solutions prove ineffective:

const tuple = getTuple() as const;

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