CODEWITHSUNDEEP
javascriptunderscore.jspass-by-reference
Michael.Lumley
Michael.Lumley

Reputation: 2385

Merge Javascript Objects and All References Pointing at Them

In javascript, variables referring to objects are passed a "copy of the reference". I am struggling with this concept as it relates to merging two objects, and the effect on the references to those objects. Attempting to use underscore's extend, take the following (coffeescript) code:

a = {
    id: 1
    type: "cat"
    name: "Angry"
}
b = {
    id: 1
    type: "dog"
    owner: "Bill"
}

c = a
d = c
e = b


_.extend(a, b)

b = a

d.home = "Nashville"

console.log(a) //Object {id: 1, type: "dog", name: "Angry", owner: "Bill", home: "Nashville"} 
console.log(b) //Object {id: 1, type: "dog", name: "Angry", owner: "Bill", home: "Nashville"}
console.log(c) //Object {id: 1, type: "dog", name: "Angry", owner: "Bill", home: "Nashville"}
console.log(d) //Object {id: 1, type: "dog", name: "Angry", owner: "Bill", home: "Nashville"}
console.log(e) //Object {id: 1, type: "dog", owner: "Bill"}

As we can see, underscore did not 'merge' the objects. Instead, what has happened is that underscore set the attributes of the object referred to by b on the object referred to by a. Later, when we replace the reference held by b to point at the object referred to by a, e continues to hold a reference to the object originally referred to by b. This is because e was not assigned a reference to b, but rather a copy of the reference held by b. Similiarly, d holds a reference, not to c but to the object referred to by a.

I would like to find a way to truly merge the two objects, so that all references to both objects point at a single destination. Is this possible in Javascript, or am I precluded from doing this as a result of Javascript's object passing structure?

Upvotes: 2

Views: 232

Answers (1)

Bergi
Bergi

Reputation: 665564

I would like to find a way to truly merge the two objects, so that all references to both objects point at a single destination. Is this possible in Javascript, or am I precluded from doing this as a result of Javascript's object passing structure?

That's impossible without changing the references, i.e. having access to all the variables and re-assigning them with the new reference value - in your case, e = b just as you did b = a.

To get the expected result, you might want to do _.extend(a, b); _.extend(b, a); so that they have the same properties - however they stay two different objects.

Upvotes: 3

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