Collections

discord.js comes with a utility class known as Collection. It extends JavaScript's native Map class, so it has all the Map features 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 use some ES6 features, so read up here if you do not know what they are.

A Map allows for an association between unique keys and their values. For example, how can you transform every value or filter the entries in a Map easily? This is the point of the Collection class!

Array-like Methods

Many of the methods on Collection correspond to their namesake in Array. One of them is find:

// 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 usually pass the parameters (value, index, array), where value is the value iterated to, index is the current index, and array is the array. For collections, you would have (value, key, collection). Here, 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:

find
filter - Note that this returns a Collection rather than an Array.
map - Yet this returns an Array of values instead of a Collection!
every
some
reduce
concat
sort

Converting to Array

Since Collection extends Map, it is an iterable, and can be converted to an Array through either Array.from() or spread syntax (...collection).

// For values.
Array.from(collection.values());
[...collection.values()];

// For keys.
Array.from(collection.keys());
[...collection.keys()];

// For [key, value] pairs.
Array.from(collection);
[...collection];

Many people convert Collections to Arrays way too much!

This can lead to unnecessary and confusing code. Before you use Array.from() or similar, ask yourself if whatever you are trying to do can't be done with the given Map or Collection methods or with a for-of loop. Not being familiar with a new data structure should not mean you default to transforming it into the other.

There is usually a reason, why a Map or Collection is used. Most structures in Discord can be identified with an id, which lends itself well to key -> value associations like in Maps.

Extra Utilities

Some methods are not from Array and are instead entirely new to standard JavaScript.

// A random value.
collection.random();

// The first value.
collection.first();

// The first 5 values.
collection.first(5);

// Similar to `first`, but from the end.
collection.last();
collection.last(2);

// Removes anything that meets the condition from the collection.
// Sort of like `filter`, but in-place.
collection.sweep((user) => user.username === 'Bob');

A more complicated method is partition, which splits a single Collection into two new Collections based on the provided function. You can think of it as two filters, but done at the same time (and because of that much more performant):

// `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); // note the "not" ! operator

Array-like Methods

Converting to Array

Extra Utilities

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