How to force proper second argument type for tthe createMessage method
const EMAIL_TEMPLATES = {
DOCTOR_SIGNUP: DoctorSignupOutput,
CHANGE_PASSWORD: PasswordParams
} as const;
@Injectable()
export class EmailService {
sendEmail() {
console.log('Email sent!');
}
createMessage(template: keyof typeof EMAIL_TEMPLATES, params: ?) {
}
}
so i.e. only objects of type DoctorSignupOutput are allowed when template equals DOCTOR_SIGNUP?
try like that
createMessage<
T extends keyof typeof EMAIL_TEMPLATES, // <- T points to a key
R extends (typeof EMAIL_TEMPLATES)[T] // <- R points to the type of that key
>(template: T, params: R) {
}
A detailed example:
interface MyInterface {
keyString: string;
keyNumber: number;
}
function createMessage<
O extends object, // object itself
K extends keyof O, // all its keys
V extends O[K] // types of keys, once K is specified, it's narrowed to related type.
>(object: O, key: K, value: V) {
console.log(object, key, value);
}
const data: MyInterface = {
keyString: 'hello',
keyNumber: 777,
};
createMessage(data, 'keyString', 'world'); // works
createMessage(data, 'keyNumber', 42); // works
createMessage(data, 'keyString', 42); // fails due to wrong type
createMessage(data, 'keyNumber', 'world'); // fails due to wrong type
createMessage(data, 'keyUnknown', 'random'); // fails due to missed key in the original interface.