mixin-in-vuex-action.js

// in mixins/trailingSlash.js
export default (url) => {
  return url.replace(/\/?$/, "/");
};

// in app.js
import trailingSlash from "./components/mixins/trailingSlash";
Vue.mixin({
  methods: {
    trailingSlash,
  },
});

// in store/actions.js
import trailingSlash from "../components/mixins/trailingSlash";
let apiUrlBase = trailingSlash(process.env.MIX_API_URL);

Nice! Thinking out loud - would the other way round work too, so reuse isn't tied to an implementation detail of mixins?

// in helpers/trailingSlash.js
export default function trailingSlash (url) {
  return url.replace(/\/?$/, "/");
};

// in app.js
import trailingSlash from "./helpers/trailingSlash";
Vue.mixin({
  methods: {
    trailingSlash,
  }
});

// in store/actions.js
import trailingSlash from "../helpers/trailingSlash";
let apiUrlBase = trailingSlash(process.env.MIX_API_URL);

Yeah, I like that better! Makes things tidier where it's used in Vuex. Nice idea 👏

Updated with suggestion from @shai126 and converted to arrow function.