TypeScript Narrowing Function Return Type Part 1

Narrowing return type of function is a little bit strange in TypeScript. So I share use cases below from bad to good 🙂

Use Cases

Usage 1 – Bad usage

Notice that even I call foo(0) in line 8, when I hover mouse on foo in line 8, popup shows val parameter type as string | number union type. But it should show as number. So it is not narrowing parameter and return type of function.

Usage 2 – Bad usage

This usage narrows parameter and return type of function as literal type. But we need to narrow to type of parameter not value of parameter as literal type. Popup shows generic parameter type as 0, that's why, parameter and return type becomes 0.

Usage 3 – Good usage (function overloads)

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TypeScript Conditional Types Good Usage Part 1

The aim of this code is simply to show related fields in data field dependent on eventType.

When ChangePassword is selected in eventType filed, then userName, oldPassword and newPassword are becoming as required fields.

When ForgotPassword is selected in eventType field, then userName is just becoming required.

const msg1 = publishMessage?.({
    eventType: 'ChangePassword',
    data: {
        userName: '',
        oldPassword: '',
        newPassword: '',
    },
    subject: '',
    topic: '',
});

const msg2 = publishMessage?.({
    eventType: 'ChangePassword_Success',
    data: {
        emailSubject: 'Password Verification code is ######',
        emailMessage: '',
    },
    subject: '',
    topic: '',
});

const msg3 = publishMessage?.({
    eventType: 'ForgotPassword',
    data: {
        userName: ''
    },
    subject: '',
    topic: '',
});

const msg4 = publishMessage?.({
    eventType: 'ForgotPassword_Success',
    data: {
        resetPasswordVerificationCode: '',
        emailSubject: '',
        emailMessage: ''
    },
    subject: '',
    topic: '',
});

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Notice that addTodoV2 and addTodoV3 error for extra field WRONG_FIELD, but addTodoV1 doesn't error for extra field in line 45.

type TodoAction =
    | { type: 'INIT' }
    | { type: 'ADD_TODO', text: string }
    | { type: 'REMOVE_TODO', id: string }
    | { type: 'SET_COMPLETED', id: string };

type KeysOfUnion<T extends { type: any }> = 
   T extends { type: infer K } ? {} extends Omit<Extract<T, { type: K }>, 'type'> ? never : K : never;

function dispatch(actionType: KeysOfUnion<TodoAction>) { throw new Error('Not implemented') };

dispatch('ADD_TODO');

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My aim is to create a function which has two parameters. First one is union value of type field in Action and second parameter should force developer to use specific object in Action.

For example, assume that we have this function function dispatch(actionType, payload){}

dispatch('SIGN_OUT', { type: 'SIGN_OUT', userName: '' });

or

dispatch('SIGN_IN_FAILURE', { type: 'SIGN_IN_FAILURE', error: '' });

type Action =
    | { type: 'INIT' }
    | { type: 'SYNC' }
    | { type: 'SIGN_UP', userName: string, password: string, attributeList: Array<any> }
    | { type: 'SIGN_IN', userName: string, pasword: string }
    | { type: 'SIGN_IN_SUCCESS', accessToken: string }
    | { type: 'SIGN_IN_FAILURE', error: string }
    | { type: 'SIGN_OUT', userName: string }
    | { type: 'FORGOT_PASSWORD', userName: string }
    | { type: 'FORGOT_PASSWORD_SUCCESS', verificationCode: string }
    | { type: 'CHANGE_PASSWORD', userName: string, oldPassword: string, newPassword: string }
    | { type: 'CONFIRM_REGISTRATION', userName: string, confirmationCode: string }
    | { type: 'RESEND_CONFIRMATION_CODE', userName: string }
    | { type: 'DELETE_USER', userName: string }
    | { type: 'GET_SESSION', userName: string }
    | { type: 'GET_SESSION_SUCCESS', session: string };

type TodoAction =
    | { type: 'INIT' }
    | { type: 'ADD_TODO', text: string }
    | { type: 'REMOVE_TODO', id: string }
    | { type: 'SET_COMPLETED', id: string };

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Creating makeWatchedObject function with overloading

type PropEventSource<T> = {
    on<K extends string & keyof T>(eventName: `${K}Changed`, callback: (newValue: T[K]) => void): void;
};

type ReturnType<T1> = T1 & PropEventSource<T1>;

function makeWatchedObject<T1>(obj1: T1): ReturnType<T1>
function makeWatchedObject<T1, T2>(obj1: T1, obj2: T2): ReturnType<T1 & T2>
function makeWatchedObject<T1, T2, T3>(obj1: T1, obj2: T2, obj3: T3): ReturnType<T1 & T2 & T3>
function makeWatchedObject<T1, T2, T3>(obj1: T1, obj2?: T2, obj3?: T3): any {

    const events = new Map<string, (a: any) => void>();

    const on_: PropEventSource<T1 & T2 & T3> = {
        on(eventName: string, callback: (newValue: any) => void): void {

            events.set(eventName, callback);
        }
    }
    const obj = { ...obj1, ...obj2, ...obj3, ...on_ };

    const proxyObj = new Proxy(obj, {
        set: function (target, property, value, receiver) {

            const event = events.get(`${property as string}Changed`);

            if (event) {
                event(value);
                return true;
            }

            return Reflect.set(target, property, value, receiver);
        }
    });

    return proxyObj;
}

Usage

const personInfo = { id: 1, firstName: 'kenan', lastName: 'hancer', age: 36 };
const personAddress = { country: 'UK', city: 'London' };
const personContact = { email: 'kh@kh.com', phone: '077111-33-33' };

let person = makeWatchedObject(personInfo, personAddress, personContact);


// works! 'newName' is typed as 'string'
person.on("firstNameChanged", (newName: string) => {

    console.log(`new name is ${newName.toUpperCase()}`);
});


// works! 'newAge' is typed as 'number'
person.on("ageChanged", (newAge: number) => {
    if (newAge < 0) {
        console.log("warning! negative age");
    }
});

person.age = 34;

Example 1

type MessageOf<T> = T extends { message: infer MessageType } ? MessageType : never;

const getMessage = <T extends { message: any }>(obj: T): MessageOf<T> => obj.message;



const sms = { phone: '07733838', message: ['Hello World', 'Merhaba Dunya'] };

const smsMessage = getMessage(sms); // string[]

const email = { from: 'kh@kh.com', to: 'kkkk@kkkk.com', message: 'Hello World' };

const emailMessage = getMessage(email); // string

Example 1.1

Notice that line 3 in below code, T extends { message: unknown } message field data type is unknown so returning value from function should be casted to any like obj.message as any

type MessageOf<T> = T extends { message: infer MessageType } ? MessageType : never;

const getMessage = <T extends { message: unknown }>(obj: T): MessageOf<T> => obj.message as any;



const sms = { phone: '07733838', message: ['Hello World', 'Merhaba Dunya'] };

const smsMessage = getMessage(sms); // string[]

const email = { from: 'kh@kh.com', to: 'kkkk@kkkk.com', message: 'Hello World' };

const emailMessage = getMessage(email); // string

Inferring Function Parameter Types

Example 1

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