group by function to replace lodash groupBy

group-by.ts

function groupBy <T = unknown> (
  list: Array<T>,
  keyGen: (item: T) => string | number,
): {[k: string]: Array<T>} {
  return list.reduce<{[k: string]: Array<T>}>((ax, dx) => {
    const key = keyGen(dx)

    if (ax[key]) {
      ax[key].push(dx)
    } else {
      ax[key] = [ dx ]
    }

    return ax
  }, {})
}