TypeScript: How to type an array of number or null elements where the first element cannot be null? [duplicate]

Question

I would like to create a valid type for an array of number where any element can be null except the first one.

My best try so far cannot handle array of any size:

type NulNum = number | null
type MyArray = 
    | [number, NulNum]
    | [number, NulNum, NulNum]
    | [number, NulNum, NulNum, NulNum]
    | [number, NulNum, NulNum, NulNum, NulNum]
    // ...


// valid cases, should compile properly
let a: MyArray = [1, null]
a = [1, 1]
a = [1, 1, null]
a = [1, 1, null, null]

// invalid cases, should trigger a compilation error
a = [null, 1]
a = [null, null]

Clément Prévost · Accepted Answer · 2020-06-11 09:04:06Z

1

Nevermind, I found the answer based on suggested linked stackoverflow questions: https://stackoverflow.com/a/56814700/2523414

type MyArray = [number, ...(number | null)[]]


// valid cases, should compile properly
let a: MyArray = [1, null]
a = [1, 1]
a = [1, 1, null]
a = [1, 1, null, null]

// invalid cases, should trigger a compilation error
a = [null, 1]
a = [null, null]

answered Jun 11, 2020 at 9:04

Clément Prévost

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