New (old kid on the block) utility TypeScript library

[Mike Austin]

https://www.npmjs.com/package/remeda

vs ramda vs rambda vs ts-belt:

https://npmtrends.com/@mobily/ts-belt-vs-rambda-vs-ramda-vs-remeda

Also, dynamically adding array element if not null came up. Maybe that is literally the name :)

  const appendIfNotNull = (array: Item["assignees"], item: Item["assignedUser"]) => [...array, ...(item ? [item] : [])];

  const assignees = pipe(
    flatMap(items, item => appendIfNotNull(item.assignees, item.assignedUser)),
    uniqueBy(assignee => assignee.id)
  );