Converting a "one or many" array to array of arrays in typescript

Question

Why doesn't the given code compile, and how do I fix it:

function f(x: string[] | string[][]): string[][] {
  return Array.isArray(x[0]) ? x : [x];
}

As far as I can tell, the return value will always be string[][] since of x is string[], x[0] will not be an array, and if x is string[][] then x[0] will be an array.

Even though, typescript doesn't recognize this and throws an array.
Is there a way to get around it without casting? How?

k.tten · Accepted Answer · 2022-09-10 18:05:08Z

2

Type predicates will narrow the type of x to string[][].

function isMatrix(v: any[] | any[][]): v is any[][] {
    return Array.isArray(v[0]);
}

function f(x: string[] | string[][]): string[][] {
  return isMatrix(x) ? x : [x];
}

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answered Sep 10, 2022 at 18:05

k.tten

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