spread argument union of tuples

user_16168082 0
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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.

typescript
2 Answers

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);
user_179525100

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;
user_95837470
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