Transforming a "singular or multiple" array into an array of arrays using TypeScript

What is causing the compilation error in the following code snippet, and how can it be resolved:

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

Upon inspection, it appears that the return value will constantly be of type string[][]. This is because if x is of type string[], then x[0] will not be an array, and if x is of type string[][], then x[0] will be an array.

Despite this, TypeScript is unable to infer the correct type and throws an error.
Is there a way to resolve this issue without type casting? If so, how?

arraystypescript

Answer №1

Type predicates can help narrow down 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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