Create a Mapped Type from a Discriminated Union

union-by.ts

// Union method to create a mapped typed from a discriminated union
export type UnionBy<
  Key extends string,
  Value extends string,
  Base,
  Map extends Record<Value, object | null>
> = {
  [TValue in keyof Map]: {
    [key in Key]: TValue;
  } & (Map[TValue] extends null ? Base : Base & Map[TValue]);
}[keyof Map & Value];

// Example usage:
export interface BugStatusMap {
  reported: { reportedDate: Date };
  inProgress: { startDate: Date };
  fixed: { fixDate: Date };
}

export interface BugBase {
  bugId: string;
  bugDescription: string;
}

export type Bug = UnionBy<'status', keyof BugStatusMap, BugBase, BugStatusMap>;

const bug = {} as Bug;

if (bug.status === 'reported') {
  // bug.reportedDate exists.
} else if (bug.status === 'inProgress') {
  // bug.startDate exists.
} else if (bug.status === 'fixed') {
  // bug.fixDate exists.
}