もう少し優れた「地図」を使用すると、より簡単に目的地にたどり着くことができます...

const myMap = new Map()

for(const file of files) {
    const [,extension] = file.name.split(".")
    if(!myMap.has(extension)) {
        myMap.set(extension, [])
    }
    myMap.get(extension).push(file)
}

// 1) Counting occurrences
const items = ['apple','apple','orange','banana','apple'];
const counts = new Map();
for (const item of items) {
  counts.set(item, (counts.get(item) || 0) + 1);
}

const users = [
  {id:1,name:'A',role:'admin'},
  {id:2,name:'B',role:'user'},
  {id:3,name:'C',role:'user'}
];
const userMap = new Map();
for (const u of users) {
  userMap.set(u.id, u);
}

const map = new Map()

for(const item of items) {
   if(!map.has(item.type)) {
       const newType = new Type(item.type, getInfoForType(item.type))
       map.set(item.type, newType)
   }
   map.get(item.type).doSomething(item)

}

class MapPlus extends Map {
    constructor(missingFunction) {
        super()
        this.missingFunction = missingFunction
    }

    get(key, createIfMissing = this.missingFunction) {
        let result = super.get(key)
        if (!result && createIfMissing) {
            result = createIfMissing(key)
            if (result && result.then) {
                const promise = result.then((value) => {
                    super.set(key, value)
                    return value
                })
                super.set(key, promise)
            } else {
                super.set(key, result)
            }
        }
        return result
    }
}

const map = new MapPlus(()=>[])

for(const item of items) {
    map.get(item.type).push(item)
}

const map = new MapPlus(()=>new MapPlus(()=>[]))
for(const item of items) {
   map.get(item.type).get(item.subType).push(item)
}

const map = new MapPlus((type)=>new Type(type, getInfoForType(type))

for(const item of items) {
    map.get(item.type).doSomething(item)
}