When using _.extend/_.assign or _.merge, you will modify the destination object.

object1 = { foo: 23, bar: 42 }
object2 = { bar: 99 }

_.extend(object1, object2) // => { foo: 23, bar: 99 }
object1 // => { foo: 23, bar: 99 }

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If you do not want to do that, simply extend into a new object:

object1 = { foo: 23, bar: 42 }
object2 = { bar: 99 }

_.extend({}, object1, object2) // => { foo: 23, bar: 99 }
object1 // => { foo: 23, bar: 42 }

Note how our _.extend statement receives {} as its first argument.