Return first object if nothing is changed?

[dantrain]

I'm looking for this behaviour:

var x = { foo: { bar: 3 }, baz: 4 };
var y = { foo: { bar: 5 } };
var z = { foo: { bar: 3 } };

var result1 = merge(x, y);
var result2 = merge(x, z);

assert(result1 === x) // => false
assert(result2 === x) // => true

Currently a new object is always created so this does not hold. Not sure if this is an unreasonable request or out of scope, I've had a look at Lodash and Ramda but can't seem to find this anywhere.

[TehShrike]

That would put deepmerge in the awkward position of having to do equality checks on everything - right now, it only compares types before copying.

Equality checks are difficult, because some people would expect object identity to be used for equality checks ({} === {} should be false) and others would only want object properties or array elements of certain primitive values compared.

I would rather push the library in the direction of always cloning everything, ala #72. It would make more sense than the current default behavior (creating a new root object, but mutating child objects on the destination object).

That way users could rely on always getting a full copy, and wouldn't have to reason about partial recursive clones.

If you do need that behavior, then I think you would be better off finding a recursive-object-equality-checking library that checks equality how you expect and doing that check before calling merge.

[dantrain]

Ok perhaps I can use a deepEqual utility first and then do a deepMerge. Not sure about the performance implications but I can see how this could be an awkward mix for this library. Thanks for the prompt reply!