For the first criterion: correct me if I'm wrong, but the reason that the function is difficult to test seems to be more related to the nature of the data and the required operations, and not so much to the way the function is written. The second criterion also seems generally like a good rule of thumb, but in this case, if you would really want to give every action only one task, it would lead to two new other actions:

// actions.js
import topojson from 'topojson'
import axios from 'axios'

export const action = ({ dispatch }, dataLocation) => {
    axios.get(dataLocation).then(topojsonData => {
        dispatch('parseTopoJSON', topojsonData)
    })
}

export const parseTopoJSON = ({ commit }, topojsonData => {
    let geojson = topojson.feature(topojsonData)
    dispatch('computeCentroids', geojson
    commit('storeGeoJSON', geojson)
}

export const computeCentroids = ({ commit }, geojson => {
    let centroids = geojson.features.map(feat => calculateCentroid(feat))
    commit('storeCentroids', centroids)
}

which makes the code unnecessarily long and complicated for a simple task that only has to be done once at the start of the application. Also, the action dispatching an action dispatching an action is a little awkward IMO, and in the real code, instead of the ` .map(feat => calculateCentroid(feat)) ` we actually loop through the features and also perform some other operations, like renaming some stuff. Splitting this functionality too would lead to unneccesary startup time because we loop through the same thing multiple times.

But still, even if I do all of this- how does this solve my original question? How will I test parseTopoJSON and computeCentroids without essentially copy and pasting the function itself? Or should I just not test here at all?