Here's a simple TS problem: I want to define multiple possible return types for a function based on the arguments of the function.

Initially, we might have a function like the following

export function formatItem(input: number): string {
  /// ..... ////
  return "final result";
}

Later on, the need arises to sometimes return an array of strings instead of a simple string. The return type is based on another function parameter:

export function formatItem(input: number, asArray = false): string {
  /// ..... ////
  if (asArray) return ["final result"];
  return "final result";
}

So how do I annotate the return type since :string is no longer valid? First place for me where I have used function overloads

export function formatItem(input: number, asArray?: false): string;
export function formatItem(input: number, asArray?: true): string[];
export function formatItem(input: number, asArray = false): string {
  /// ..... ////
  if (asArray) return ["final result"];
  return "final result";
}

A detail worth noting is that the order of the overload statements matters. Since asArray = false is the default here, the return value caused by this should be marked first so that in const result = formatItem(99) the resulting variable is correctly interpreted as a string.