Discord.js comes with this utility class known as
Map class, so it has all the features of
Map and more!
If you're not familiar with
Map, read MDN's page on it before continuing. You should be familiar with
Array methods as well. We will also be using some ES6 features, so read up here if you do not know what they are.
Map allow for an association between unique keys and their values, but lack an iterative interface.
For example, how can you transform every value or filter the entries in a
This is the point of the
Many of the methods on
Collection are based on their namesake in
Array. One of them is
// Assume we have an array of users and a collection of the same users. array.find(u => u.discriminator === '1000'); collection.find(u => u.discriminator === '1000');
The interface of the callback function is very similar between the two.
For arrays, callbacks are usually passed the parameters
(value, index, array), where
value is the value it iterated to,
index is the current index, and
array is the array itself. For collections, you would have
(value, key, collection).
value is the same, but
key is the key of the value, and
collection is the collection itself instead.
Methods that follow this philosophy of staying close to the
Array interface are as follows:
filter- Note that this returns a
Collectionrather than an
map- Yet this returns an
Arrayof values instead of a
Converting to Array
There are two ways you might want to convert a
Collection into an
Array. The first way is the
They simply create an array from the items in the collection, but also caches it too:
// Not computed again the second time, it is cached! collection.array(); collection.array(); // Any change to the collection, however, invalidates the cache. // This call to `array` must be recomputed. collection.delete('81440962496172032'); collection.array();
This caching behavior is undesirable if you are planning to mutate the array, so instead, you can use
// For values. Array.from(collection.values()); // For keys. Array.from(collection.keys()); // For [key, value] pairs. Array.from(collection);
Many people use
array way too much! This leads to unneeded caching of data and confusing code. Before you use
array or similar, ask yourself if whatever you are trying to do can't be done with the given
Collection methods or with a for-of loop.
Some methods are not from
// A random value. Be careful, this uses `array` so caching is done. collection.random(); // The first value. collection.first(); // The first 5 values. collection.first(5); // Similar to `first`, but from the end. This uses `array`. collection.last(); collection.last(2); // Removes from the collection anything that meets a criteria. // Sort of like `filter`, but in-place. collection.sweep(user => user.username === 'Bob');
A more complicated method is
partition, which splits a collection into two, based on a certain criteria.
You can think of it as two
filters, but done at the same time:
// `bots` is a Collection of users where their `bot` property was true. // `humans` is a Collection where the property was false instead! const [bots, humans] = collection.partition(u => u.bot); // Both return true. bots.every(b => b.bot); humans.every(h => !h.bot);