A slightly better 'Map' can get you there more easily...

In JavaScript the Map is a very useful built in class that creates an O(1) lookup between a key and a value.

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)
}

You can use Maps for all sorts of things you are likely to do regularly:

• Creating grouped lists of data, like the above example grouping by file extension

• Aggregating data, like counting or summing values across a range of keys
  

// 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);
}

• Creating rapid lookups to be used in subsequent steps

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);
}

Map is preferred to using a simple object ({}) for a couple of reasons, so long as you don't have to store the result using a stringify:

• It can take keys which are not strings
• It's slightly faster than an Object even if you are using string keys

There can be a lot of boilerplate and mixed concerns though, if the object you are storing in the map needs construction, which is anything from a simple array to a complex object, this needs to be interspersed with the code that uses it.

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)

}

This "can" be ok, but it becomes harder to keep DRY if you need to update or initialise the value in multiple places.

For this reason I use a MapPlus class, which is an extension to Map that provides a missing key initialiser function that can be supplied to the constructor or as a second parameter to the get if the initialiser does need in context information beyond just the key.

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
    }
}

With this you can just do things like:

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

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

As if the key is missing it will just make an empty array.

I often need two levels of this so I'll have maps defined like this:

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

The constructor function does get the key being used so we can also do:

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

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

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