Initialize array with one element, or push element if array exists

Question

Let's say you have a hash o of arrays; for example registered callbacks for events, when each event can have 0 or more callbacks.

Is there a better way to say this in ES6?

if (key in o) o[key].push(x); else o[key] = [x]

By "better" I mean more easily understood by other developers. Possibly more concise, but not at the expense of readability. A particular (common) problem is that o is often a longer expression, e.g. this.listeners, and repeating it three times seems suboptimal. So real code might look like this:

if (event in this.listeners)
    this.listeners[event].push(callback);
else
    this.listeners[event] = [callback];

Answer 1

By using the logical nullish assignment ??=, you could check the property and if it is undefined or null, create a new array.

(o[key] ??= []).push(x);

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Older approach:

You could use a logical OR and create an array if necessary.

o[key] = o[key] || [];
o[key].push(x);