# 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!

WARNING

If you're not familiar with Map, read MDN's page on it (opens new window) before continuing. You should be familiar with Array methods (opens new window) 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

There are two ways you might want to convert a Collection into an Array. The first way is the array or keyArray methods. They create an array from the items in the collection but also caches it:

// Not computed 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 Array.from:

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

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

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

WARNING

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 Map or Collection methods or with a for-of loop.

# Extra Utilities

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

// 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 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);

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