unit-testing case use new Vue(xx) and new Vue.extend(xx)

[zerofront]

Hi,
https://cn.vuejs.org/v2/guide/unit-testing.html#ad
In chapter unit-testing, I was confused about
new Vue(myComponent)
and
new Vue.extend(myComponent)
what 's the different between them ??

but in unit-testin case, I had try to changed them. but it seem to be same.