Proper way of creating an array or pushing into it if it already exists

In my code I have an element, and I want to have it in an array: either the array already exists and then I push my element, or it doesn’t and in that case I create the array with this only element:

if (arr) {
  arr.push(elt);
} else {
  arr = [elt];
}

I’m sure there is a better way to do this. I looked into spreading and nullish coalescing operator, but I didn’t find any correct syntax.
Does anybody have an idea on how I could write my lines with a nicer syntax?
Thanks in advance.

>Solution :

You can use the expression arr || [] to return an array which is either arr if arr is defined already or [] otherwise. Then you can simply concat the new value to it:

arr = (arr || []).concat([elt])

Note the new value needs to be enclosed in [] to prevent elt values which are arrays from being flattened.