typescript, Array.prototype.map() has error 'expression is not callable' when the callee array is union with empty array

As of December 2020 there is an open issue about this: https://github.com/microsoft/TypeScript/issues/36390

It probably won't get fixed for a little while (Until TypeScript 4.2 or later).

In the mean time you can add as any[] to get rid of error:

const arr: number[] | string[] = [];
// Add as any[]
(arr as any[]).map((a: number | string, index: number) => { 
    return index
});