We have a function that modifies a JS object, by adding some custom properties t

Question

We have a function that modifies a JS object, by adding some custom properties to it. The function doesn't return antyhing

addTransaction: function (obj) {
     obj.transactionId = this.getTransactionId;
     obj.id = this.recordId;
},
Somebody said they preferred that addTransaction return the obj.

Here's what I thought

If I don't return anything (and document that the object is going to be modified), it's kind of clear that the object is going to be modified, as if the name were addTransactionToObj

If I do want to add a return value, I shouldn't modify the incoming object, I should clone the given object, add my properties to the clone and return the clone.

Having a return value that just returns one of the (modified) parameter just sounds wrong
Does anybody have a preference in this matter?

Explanation / Answer

It allows you to do method chaining, which a lot of people feel improves readability. It's a very common idiom in JavaScript, which means a lot of people expect it, especially if the rest of the code base is similar. Your second point about cloning the object also has merit, if used to make your object immutable. How beneficial that is depends on your specific application.

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