jQuery/Underscore/Lodash extend, without the mutation

I was writing a little Javascript today where I wanted to scale a bunch of coordinate data from a source object to a new set of scaled objects. So I did something like this...

var originalData = JSON.parse(source);
var scaledData = _.extend(
    originalData,
    {objects: _.map(toScaledObject, originalData.objects)}
);

But surprisingly didn't get the results I expected. Turns out firstly I had the arguments to map the wrong way around, but before realising this I also noticed that my original source object was also getting updated. Wat?!?

Destination Object

After checking the docs I realised both jQuery and Underscore/Lodash refer to the first object passed to the extend function as the destination object. So while the function also (curiously) returns this destination object, it is in fact mutated in-place as part of the extend operation.

The Workaround

The workaround I used was to provide a new object for this destination object, and then as part of extend map the source object onto this. Like...

var originalData = JSON.parse(source);
var scaledData = _extend(
    {},
    originalData,
    {objects: _.map(originalData.objects, toScaledObject)}
);

Simple enough. So library writers please try to be immutable by default.