Is it possible to conditionally add an element to a possibly undefined array on an object in one line in JS?

Issue

I have an object with ids for keys that I want to conditionally add an array to. I could do this, but I don't want this code repeated 3 times in my function:

const myObj = {};
const myKey = '1233';
const myValue = true;

if (!(myKey in myObj)) {
    myObj[myKey] = [];
}
myObj[myKey].push(myValue);

I would like to do something myObj.upsert(myKey, myValue), but modifying the Object prototype currently causes performance issues. Using Object.create (as suggested) causes a lot of TypeScript issues in my application and I was casting my object back to one without the upsert method to pass it along which felt bloated and wrong.

So I opted to use the nullish coelescing assignment operator in my code:

const myObj = {};
const myKey = '1234';
const myValue = true;

myObj[myKey] ??= [];
myObj[myKey].push(myValue);

and that works great! But, is it possible to do this in one line?

I was thinking something like this might work, but no:

myObj[myKey] ??= [...myObj[myKey], myValue]

Solution

The nullish-coalescing assignment operator forms an expression. You don't have to put it in a separate statement (line):

(myObj[myKey] ??= []).push(myValue);

Answered By - Bergi