TypeScript Version: playground / 3..3.333

Search Terms:
union type, array map

Code

function flattenStringArrays(values: string | string[] | string[][]): string{

    if (Array.isArray(values)) {
        return values.map(value => flattenStringArrays(value)).join("");
    } else {
        return values;
    }
}

Expected behavior:
here the value in the map expression should be of type string[] | string[][]

Actual behavior:
Compilation error thrown:

Cannot invoke an expression whose type lacks a call signature. Type '((callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => U[]) | ((callbackfn: (value: string[], index: number, array: string[][]) =>
U, thisArg?: any) => U[])' has no compatible call signatures.

Playground Link:
link

Related Issues: