typescript error indicating a missing property that is expected to exist

I am struggling to comprehend the reason behind this error message.

Property 'message' does not exist on type '{ username: string; } | { message: string; }'.

The following is a snippet of the relevant code:

// types.ts
type LoginMessage = {
  type: 'login';
  payload: { username: string };
};

type SendChatMessage = {
  type: 'send_chat_message';
  payload: { message: string };
};

export type ClientToServerMessage = LoginMessage | SendChatMessage;

// reducer.js

const { message } = action.payload; // this specific line is causing the issue
// action is of type ClientToServerMessage

Answer №1

The solution eluded them... a crucial break; was absent in a switch case

Answer №2

To narrow the ClientToServerMessage union type, you'll have to implement a user-defined type guard.

An example provided in the documentation demonstrates how these type guards can be beneficial for narrowing a union type.

function isFish(pet: Fish | Bird): pet is Fish {
    return (pet as Fish).swim !== undefined;
}

When implementing conditional branches in your code, the type will be narrowed to Fish if swim is defined.

if (isFish(pet)) {
    pet.swim(); // the compiler recognizes that `pet` is of type `Fish`
}

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