spread argument union of tuples

Question

I have function which returns union type of tuples. I want to pass it to function which accepts all its forms using 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 on last line typescript compiler complains:

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

I do not want to redefine calc function. I need function getTuple to return union type which supports spread operator.

Answer 1

you can use variadic parameter to calc() function and passing the 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 reason this is occurring is because the calc function has fixed parameters. However, tuple is an array which can change.

One way you can fix this is by declaring a type with tuple at its declaration (this is what the error is telling you to do):

const tuple: TupleUnion = getTuple();

Another way would be too to add the type to the calc function and remove the fixed parameters:

function calc(...args: TupleUnion)

You would then be able to access the contents of args with array syntax just like you would a tuple:

args[0]

A third way is to assert tuple as a constant, but this is hacky and should be considered if nothing else works:

const tuple = getTuple() as const;