交叉点“……”被缩减为从不

Question

I receive an event from an API with the type of event assigned to it. For each type of event there is a different schema, so I created a separate type for every event. The problem is, that Typescript reduces an intersection of events to never , because the property type is different for every event type. What should I do to avoid this error?

Here's a simplified code:

type EventType = 'login' | 'newMessage';

type ExampleEvent = {
  type: EventType;
  payload: unknown;
};

type LoginEvent = ExampleEvent & {
  type: 'login';
  payload: {
    userId: string;
  };
};

type NewMessageEvent = ExampleEvent & {
  type: 'newMessage';
  payload: {
    messageId: string;
  };
};

const loginEvent = {
  type: 'login',
  payload: {
    userId: 'abc',
  },
};

const newMessageEvent = {
  type: 'newMessage',
  payload: {
    messageId: 'abc',
  },
};

const events = [loginEvent, newMessageEvent];

const handlers = {
  login: (event: LoginEvent) =>
    console.log('user logged in', event.payload.userId),
  newMessage: (event: NewMessageEvent) =>
    console.log('new message', event.payload.messageId),
};

events.forEach(event => {
  // Argument of type '{ type: string; payload: { userId: string; }; } | { type: string; payload: { messageId: string; }; }' is not assignable to parameter of type 'never'.
  // The intersection '...' was reduced to 'never' because property 'type' has conflicting types in some constituents.
  // Type '{ type: string; payload: { userId: string; }; }' is not assignable to type 'never'.
  handlers[event.type as EventType](event);
});

Answer 1

Since you create your event types with using intersection, so your left side of your handler definition can be anything intersected with ExampleEvent so you need to define your handler type like this:

const handlers: {
  [K in EventType] : (event: ExampleEvent & any) => void
} = {
  login: (event: LoginEvent) =>
    console.log('user logged in', event.payload.userId),
  newMessage: (event: NewMessageEvent) =>
    console.log('new message', event.payload.messageId),
};

To make event to accept types LoginEvent and NewMessageEvent

Playground