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Just getting back into doing a lot of Javascript and trying to understand Javascript objects better. I asked previous question earlier but this one is different. Can I disconnect a reference relationship that exists between objects?

For example:

var objA={};
objA.my_name='Joe';
var objB=objA;
objB.my_name="jake"; // objA.my_name="jake"
objB.something="this is something";
objA.last_name="Jackson";
console.log(objA.something);  // "this si something" can add to parent object at runtime
console.log(objB.last_name);  // "Jackson" can add to child object at runtime
// now I'd like to cut off objB from objA such that:
objB.cell_phone='323-213-2323';
console.log(objA.cell_phone); // '323-213-2323' but would like undefined; would like this behavior

thx in advance

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  • 1
    You mean, how to clone an object? Commented Jan 21, 2013 at 18:31
  • no, I'd like reference behavior at beginning but then be able to tell it to stop. I looked at the cloning answers but would like reference behavior at start and then disconnect Commented Jan 21, 2013 at 18:32
  • When you do this: var objB = objA; you end up with just one object; it's one object with two separate variables pointing at it. There's no "lazy clone" mechanism in JavaScript. Commented Jan 21, 2013 at 18:34
  • @timpone so you mean, how to clone an object later on? Or, you want two objects that will temporarily mirror each other's state? Commented Jan 21, 2013 at 18:36
  • the latter. basically create a reference relationship and then, at a later point, be able to disconnect the relationship. I think the answer below gets at it which is that we can't really do real metraprogramming in Javascript this way (maybe some other way) Commented Jan 21, 2013 at 18:39

1 Answer 1

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You can't tell an object to stop having reference behavior. That's the way javascript works.

You can copy an object so that you create a totally new object with the same properties as the original, but a completely separate object that won't have reference behavior.

A shallow copy can be made by simply iterating over the properties of an object and assigning each one to a new object.

function shallowCopy(src, dest) {
    for (var prop in src) {
        if (src.hasOwnProperty(prop)) {
            dest[prop] = src[prop];
        }
    }
}

A shallow copy of an array can be done like this:

var arr = [1,2,3];
var copyArr = [].slice.call(arr, 0);

Deep copies where properties that are objects or arrays themselves are also copied requires more work because you essentially have to check if they are objects or arrays and then recursively copy them and their contents. Shallow copies, as shown here, are much easier and, if you know the contents of your data are often sufficient.

If you want to look at an advanced version of copying (including deep copying), you can see the code for jQuery's .extend() function here. Follow that link and search for fn.extend.

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10 Comments

It's not just "simply iterating over the properties", if properties themselves are objects or arrays of objects you need recursion.
Too bad there's no easy and generic way to make a deep copy of any object.
@Christophe - that's why that sentence says "shallow copy".
Note that if arr is a real array, then arr.slice(0) will do as well.
@JanDvorak - yeah, I'm in the habit of doing it this way so it works on the various pseudo-arrays that appear occasionally (such as collections returned from DOM calls or the arguments pseudo-array).
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