object properties overriding type issue

Question

I am trying to create a function in typescript to override the default values of an object with the values given in another object.

Basically it should take in 2 objects and add the non-null properties of 2nd object to the first object and return the modified obj. I could use destructuring like this {...a, ...b} but then if there are any null values in the b object, those would override the values in a object.

Here is a function I wrote, unfortunately, can't understand what typescript is complaining about.

function assignIfPresent<T extends Record<string, unknown>>(origObj: T, additions:Partial<T> ) {
    for (const propName in additions) {
        if (additions[propName])
            origObj[propName] = additions[propName];
    }

    return origObj;
}

error at origObj[propName]

const propName: Extract<keyof T, string>
Type 'T[Extract<keyof T, string>] | undefined' is not assignable to type 'T[Extract<keyof T, string>]'.
  Type 'undefined' is not assignable to type 'T[Extract<keyof T, string>]'.

UseCase:

class User {
    public firstName!: string;
    public lastName!: string;
    public gender?: string;
}

const newUser = new User();

newUser.lastName = "Biden"

const userDetails: Partial<User> = {
    firstName: "Joe",
    lastName: undefined,
}

console.log(assignIfPresent(newUser, userDetails)) //[LOG]: { "lastName": "Biden", "firstName": "Joe" } 
console.log({...newUser, ...userDetails}) //[LOG]: { "firstName": "Joe" }

would be great if anyone can help me out.

MoxxiManagarm · Accepted Answer · 2020-09-09 10:04:59Z

1

I suggest du use lodash's pickBy to remove falsy values

import { pickBy } from 'lodash';

class User {
    public firstName!: string;
    public lastName!: string;
    public gender?: string;
}

const newUser = new User();

newUser.lastName = "Biden"

const userDetails: Partial<User> = {
    firstName: "Joe",
    lastName: undefined,
}

console.log({...newUser, ...pickBy(userDetails, _ => !!_)})

See stackblitz: https://stackblitz.com/edit/typescript-vthhuy?file=index.ts

answered Sep 9, 2020 at 10:04

MoxxiManagarm

9,2214 gold badges20 silver badges51 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Auston Barboza Over a year ago

This solution works. I checked it out on stackbiltz. Thank you so much. I would love to know what is the cause of the error too. Would be great if you can help me understand it.

MoxxiManagarm Over a year ago

Because the loop also includes undefined. for(const propName in { lastName: 'foo' }) console.log(propName) prints lastNameand undefined, for(const propName in { }) console.log(propName) prints just undeifned.

Auston Barboza Over a year ago

I tried executing

for (const propName in {lastName:"bkjsdb"}) console.log((propName === undefined || propName === null) ? "undefined1" : propName);

. I was not able to get undefined to be printed on the console. Am i missing something here? please correct me if i am wrong.

Collectives™ on Stack Overflow

object properties overriding type issue

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related