Typescript - require single literal value in N-length array

Question

Is the following somehow possible? I want to require the 'foo' literal, but allow the array to be any shape (i.e. I can't use an X-length tuple with pre-defined positions)

type requireFoo = ???

const works: requireFoo = ['bar','foo'] //Should work
const notWork: requireFoo = ['tar', 'bar'] //Should not work

Here's a playground template

If there's a problem in particular that you're trying to solve, you might want to post that as a question instead so that we might be able to provide a more ergonomic solution by changing both the javascript and the typings. — Rubydesic
– Rubydesic, Commented Jan 20, 2021 at 17:54

Rubydesic · Accepted Answer · 2021-01-20 17:49:49Z

2

It's a strange requirement. If you require foo occurs at the end or start of the tuple you can do this:

type requireFoo = [...any[], 'foo']

const works: requireFoo = ['bar', 'foo' as const] // Good 
const notWork: requireFoo = ['tar', 'bar' as const] // Bad

note: Rest arguments in the start of the tuple as shown above are only available in typscript 4.2 beta.

answered Jan 20, 2021 at 17:49

Rubydesic

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