immutable.js filter and mutate (remove) found entries?

Question

I have two loops, one for each day of the month, other with all events for this month. Let's say I have 100 000 events. I'm looking for a way to remove events from the main events `List` once they were "consumed". The code is something like: const calendarRange = [{initialDate}, {initialDate}, {initialDate}, {initialDate}, ...] // say we have 30 dates, one for each day const events = fromJS([{initialDate}, {initialDate}, {initialDate}, ...]) // let's say we have 100 000 calendarRange.map((day) => { const dayEvents = events.filter((event) => day.get('initialDate').isSame(event.get('initialDate'), 'day')) // we get all events for each day doSomeThingWithDays(dayEvents) // how could I subtract `dayEvents` from `events` in a way // the next celandarRange iteration we have less events to filter? // the order of the first loop must be preserved (because it's from day 1 to day 3{01}]) } With lodash I could just do something like: calendarRange.map((day) => { const dayEvents = events.filter((event) => day.get('initialDate').isSame(event.get('initialDate'), 'day')) // we get all events for each day doSomeThingWithDays(dayEvents) pullAllWith(events, dayEvents, (a, b) => a === b) } How to accomplish the same optimization with immutablejs? I'm not really expecting a solution for my way of iterating the list, but for a smart way of reducing the events `List` in a way it get smaller and smaller..

Answer 1

Hey!

The Core Challenge

Immutable.js structures are, well, immutable — you can't mutate them in place like Lodash's pullAllWith. But you can reassign a variable to a progressively smaller Immutable List on each iteration. The key is using let instead of const and leveraging .filterNot().

The Immutable.js Solution

let remainingEvents = fromJS([...]) // 100,000 events — use `let`, not `const`

calendarRange.map((day) => {

const dayEvents = remainingEvents.filter((event) =>

day.get('initialDate').isSame(event.get('initialDate'), 'day')

)

doSomeThingWithDays(dayEvents)

remainingEvents = remainingEvents.filterNot((event) =>

day.get('initialDate').isSame(event.get('initialDate'), 'day')

)

})

Why This Works

.filterNot() is the Immutable.js equivalent of "remove these entries." It returns a new List with the matched events excluded, and by reassigning remainingEvents each loop, every subsequent day iterates over a smaller and smaller dataset — exactly the optimization you're after.

Even Better: Pre-Group with .groupBy()

If you really want to eliminate redundant passes altogether, pre-bucket your events once upfront:

const eventsByDay = events.groupBy((event) =>

event.get('initialDate').format('YYYY-MM-DD')

)

calendarRange.map((day) => {

const dayKey = day.get('initialDate').format('YYYY-MM-DD')

const dayEvents = eventsByDay.get(dayKey, List())

doSomeThingWithDays(dayEvents)

})

This is O(n) total instead of O(days × events). You build the map once, then each day lookup is O(1). For 100,000 events across 30 days, this is the real win.

TL;DR

Approach How
Shrinking list (your pattern)	let + .filterNot() reassignment
Best performance overall	Pre-group with .groupBy()

The groupBy approach is what I'd recommend in production — it respects day order (since you still iterate calendarRange in order) and avoids re-scanning the list entirely.

Let me know if you have any other questions.