Adding elements to Immutable List within forEach - immutable.js

Question

I have an Immutable Map, where each key is an arbitrary category (these will be a List). I then have an array of strings that each category must have. If a specific category does not contain that string, it will be added to its respective List inside the Map.
const categories = new Map({
cat1: new List(['animal', 'color']),
cat2: new List(['animal']),
});
const missingKeys = new Map({
cat1: new List(),
cat2: new List(),
});
categories.keySeq().toArray().forEach((category) => {
const keys = categories.get(category).keySeq().toArray();
const requiredKeys = ['animal', 'color'];
// loop through required keys and push any key that is not found
// in the categories Map
requiredKeys.forEach((key) => {
if (keys.indexOf(key) === -1) {
// add missing keys to respective category of `missingKeys`
missingKeys.get(category).push(key)
}
});
});
I expect that after the loop completes, categories will be updated accordingly. However, when I try to console.log categories, the Map has not been updated.

Answer 1

You seem to be missing the whole "immutable" part of this library. The line
missingKeys.get(category).push(key)
does not mutate missingKeys or any of the lists it contains. It returns a new List with the missing key at the end.
If you want missingKeys to change, you need to reassign the variable:
missingKeys = missingKeys.update(category, keyList => keyList.push(key));
Don't forget to change that
As a side note, you're doing a lot of weird conversion with keySeq and toArray. What you have could be written more simply as:
const requiredKeys = ['animal', 'color'];
categories.forEach((keys, category) => {
requiredKeys.forEach(requiredKey => {
if (!keys.contains(key)) {
missingKeys = missingKeys.update(category, keyList => keyList.push(key));
}
});
});
Or the even more idiomatic functional way:
const requiredKeys = List(['animal', 'color']);
const missingKeys = categories.map(keys =>
requiredKeys.filter(key =>
!keys.contains(key)))