RequestMessage.ts

interface MessageMap {
    method1: { prop1: string };
    method2: { prop2: string };
    method3: { prop3: string };
}

export type RequestMessage<T> = {
    [P in keyof T]: { action: P, data: T[P] }
}[keyof T];

const x: RequestMessage<MessageMap> = null;

switch (x.action) {
    case 'method1':
        var p1 = x.data.prop1;
        break;
    case 'method2':
        var p1 = x.data.prop2;
        break;
}