underscorejs - what is the difference between extendOwn vs extend?

Question

While going through underscorejs's list of methods, I couldn't help but notice a method which I don't remember being there before: extendOwn

The documentation for this method says the following:

extendOwn _.extendOwn(destination, *sources) Alias: assign

Like extend, but only copies own properties over to the destination object.

I understand how .extend() is used and what it does... but for the life of me I cannot understand how it differs from .extendOwn().

I tried using .extend() and then .extendOwn() to extend a few objects just to see if maybe there was something obvious that would happen - but they seem to both produce the same result.

var a = {
    foo: false
};

var b = {
    bar: true
};

// This will produce { foo: false, bar: true }; ..just like _.extend() would =\
_.extendOwn( a, b );

Any insight into this mystery would be greatly appreciated!

Answer 1

"Own properties" is a technical term in JS. An object's own properties are ones that it didn't inherit.

Here's a short snippet that exposes the different behavior of extend and extendOwn:

// lines have length
line = { length: 4 }

// planes have width and inherit length
plane = Object.create(line)
plane.width = 5
plane.length  // 4

// making a cube object, using extend
cube = _.extend({ height: 6 }, plane)
cube.length  // 4

// making a cube object, using extendOwn
notACube = _.extendOwn({ height: 6 }, plane)
notACube.length  // undefined

As you can see, extendOwn only copied properties that were defined directly on the source, whereas extend also copied the ones defined along its prototype chain. Also note the symmetry with _.has:

_.has(plane, 'width')   // true
_.has(plane, 'length')  // false