Typescript: type arrays with different type for each element

Question

I'd like to define a typed array, specifying a type for the first element, another type for the second and so on...

In fact, I'm trying to get rid of the following casts:

const cases = [
  [ 'http://gmail.com', false ],
  [ '/some_page', true ],
  [ 'some_page', false ]
]

describe("'isInternalLink' utility", () => {
  test.each(cases)(
    "given %p as argument, returns %p", (link, result) => {
      expect(
        isInternalLink(<string>link)
      ).toEqual(
        <boolean> result
      )
    }
  )
})

As you can see, each element in cases is an array which has a string as first element and a boolean as second...

Just to be clear, I don't want an array of string | boolean type, I want an array whose first element is a string and whose second element is a boolean.

Any idea how to improve this???

Interesting, as const doesn't work right in this case (at least in my vscode). I wonder if the typing for test.each needs some work? @TitianCernicova-Dragomir — Brian Adams
– Brian Adams, Commented Sep 9, 2019 at 3:42
I get this error: Type 'readonly ["gmail.com", false]' is not assignable to type 'string'. What a shame, that would have been the perfect solution to this problem — opensas
– opensas, Commented Sep 9, 2019 at 3:43

Brian Adams · Accepted Answer · 2019-09-09 03:23:50Z

6

cases is an Array, and each element in cases is a Tuple, so if you define cases like this:

const cases: [string, boolean][] = ...

...or alternatively like this:

const cases: Array<[string, boolean]> = ....

...then link will be a string and result will be a boolean and your test can be simplified to this:

describe("'isInternalLink' utility", () => {
  test.each(cases)(
    "given %p as argument, returns %p", (link, result) => {
      expect(isInternalLink(link)).toEqual(result)
    }
  )
})

answered Sep 9, 2019 at 3:23

Brian Adams

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2 Comments

opensas Over a year ago

YES, exactly what I was looking for (too bad the as const option wouldn't work in this scenario...)

Ooker Over a year ago

can I use this while inside another interface?

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