feast/src/main.ts

import Vue from 'vue'
import App from './App.vue'

declare global {
  interface Array<T> {
    joinGeneral (item: T, endItem: T|null): Array<T>;
  }
}

/* eslint-disable-next-line */
Array.prototype.joinGeneral = function (item, endItem = null) {
  if (endItem === null) {
    return this.slice(0, -1).flatMap(x => [x, item]).concat(this.slice(-1))
  } else {
    return this.slice(0, -2).flatMap(x => [x, item]).concat(this.slice(-2, -1).flatMap(x => [x, endItem])).concat(this.slice(-1))
  }
}

Vue.config.productionTip = false

new Vue({
  render: h => h(App)
}).$mount('#app')