Type definition: An array of a certain type at least with e.g. 2 elements

Question

Maybe that's a stupid question, but I would like to know if there is a possibility to say, "foo" must have at least 2 array elements of type "Foo"?

let foo: Array<Foo>; // how to say at least 2 Foos?

Answer 1

Yes, you can use a rest element in a tuple type:

let foo: [Foo, Foo, ...Foo[]];

And the compiler will enforce it:

declare const f: Foo;
foo = [f, f, f]; // okay
foo = [f, f]; // okay
foo = [f]; // error, property "1" is missing
foo = []; // error, properties "0" and "1" are missing

Of course, the compiler won't easily be able to verify that some arbitrary Foo[] matches this, even if you do the "obvious" length check:

declare const fooArray: Foo[];
foo = fooArray; // error, can't verify
if (fooArray.length >= 2) {
  foo = fooArray; // error, still can't verify
}

So you might find yourself in need of something like a user-defined type guard function to convert a Foo[] to a [Foo, Foo, ...Foo[]]:

function isAtLeastTwoFoos(x: Foo[]): x is [Foo, Foo, ...Foo[]] {
  return x.length >= 2;
}

And then you can test that way.

if (isAtLeastTwoFoos(fooArray)) {
  foo = fooArray; // okay
}

Hope that helps; good luck!

Link to code