conditional_type.ts

export enum MemberActionOptions {
  DoThis,
  DoThat,
  OtherThing
}

export type MemberAction = {
  type: MemberActionOptions.DoThis,
  thisThing: number,
} | {
  type: MemberActionOptions.DoThat,
  thatThing: number,
} | {
  type: MemberActionOptions.OtherThing,
  other: number,
}

let action:MemberAction;

action = {type:MemberActionOptions.DoThis, thisThing:12}; //VALID
action = {type:MemberActionOptions.DoThat, thatThing:12}; //VALID

action = {type:MemberActionOptions.DoThat, thisThing:12}; //COMPILE ERROR