# 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!

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 iterableopen in new window, 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];

# 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:

// `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);