Is there a purely functional way to merge objects in JavaScript?

Question

Say I have an object with some properties:

const obj = {
  key1: 1
  key2: 2
}

and I have a function someFunc that takes one object as a parameter. I want to pass obj with some additional parameters, like

someFunc({
  key1: 1
  key2: 2
  otherKey: 3
})

The problem is that I don't want to actually change the state of obj. After someFunc has been called it should not contain otherKey as a property.

This leaves Object.assign out of the question:

let ident = someObj => someObj;

ident(Object.assign(obj, {otherKey: 3}))
console.log(someObj); // -> {key1: 1, key2: 2, otherKey: 3}

Creating a new variable and assigning otherKey = 3 to it does the same thing.

I know I can use some sort of deep duplication - but that seems like an unnecessarily complex way of handling a fairly simple problem.

The most elegant way of doing this would be a pure function that takes some objects obj1, obj2, returns the same output as Object.assign(obj1, obj2), but alters neither of them. Is there some obvious way of doing this in JS that I haven't seen, or do I have to use some other library?

Answer 1

Just reverse the parameters to Object.assign - it will add the properties from obj to a new object:

ident(Object.assign({otherKey: 3}, obj))

Caution

But you must be careful about properties that are already present in obj as they will overwrite the ones in the new array.