Is there an immutable way of assigning a key to an object?

Question

I'm looking for a way to do the following in an immutable style, i.e., not needing to instantiate an empty object a first.

let a = {};
let append = '123'
let keyName = `myKey${append}`
a[keyName] = 'hello world';

// result
//a = {
//  myKey123: 'hello world'
//}