commit Vuex mutation synchronously after data manipulation inside store action

In my Vuex store i have a action which once get result, loop over each element of the result and add's new property to each one.

Vue.axios
  .get(endpoint, headers)
  .then(res => {
    if (Array.isArray(res.data) && res.data.length) {
      res.data.forEach(el => {
          el.link = 'some_dynamic_link'
      })
      commit('SET_ACTIVITY', res.data)
    }
  })

the problem is that my store ends up with the data from rsult, WITHOUT the link added to any of the elements. i thought that something like this tweak will help:
if (_.has(res.data[res.data.length - 1], 'link')) { commit('SET_ACTIVITY', res.data) } but then the mutation just ends up not executing at all, any idea how can i achieve this synchronous behavior ?

update tried to use map instead:

.then(res => {
  if (Array.isArray(res.data) && res.data.length) {
    commit('SET_ACTIVITY', res.data.map(el => {
        el.link = 'some_dynamic_link'
    }))
  }
})

but than the store contains activities array where elements are undefined's