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4.1.1 • Public • Published

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This is a curated, open-source project of common JavaScript collections with full support for TypeScript. Initially started as a side-project to abstract away some common patterns in other projects, this library continues to grow to become a full standard library of efficient algorithms.

npm i scl

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📖 Go straight to the documentation!


Using the priority queue to sort some tasks on importance

import { PriorityQueue } from "scl"

interface Task {
 priority: number
 description: string

const tasks = new PriorityQueue<Task>({
  compare: (a, b) => a.priority < b.priority

tasks.add({ description: 'Do the dishes', priority: 5 })
tasks.add({ description: 'Buy food', priority: 1 })
tasks.add({ description: 'Play some games', priority: 52 })
tasks.add({ description: 'Go for a walk', priority: 10 })
tasks.add({ description: 'Program like crazy', priority: 20 })

// Take the most important task from the queue
const buyFood = tasks.pop();

// See what the next task looks like without removing it
const doTheDishes = tasks.peek()

console.log('I should do the remaining tasks in the following order:');
for (const task of tasks) {
  console.log(`- ${task.description}`);

This will output the following text:

I should do the remaining tasks in the following order:
- Do the dishes
- Go for a walk
- Program like crazy
- Play some games

Sorting and querying a list of people based on their age

import { TreeIndex } from "scl"

interface Person {
  name: string;
  email: string;
  age: number;

const people = new TreeIndex<Person, string>([
    name: 'Bob',
    email: '',
    age: 45,
    name: 'Fred',
    email: '',
    age: 33,
    name: 'Lisa',
    email: '',
    age: 37,

// Lisa is the oldest person who is at the very most 40 years old.
const lisa = people.getGreatestLowerBound(40);

// Bob is the youngest person older than Lisa
const bob =;

// No one is older than Bob
assert( === null);

Storing many different translations in the same dictionary

import { TreeMultiDict } from "scl"

const d = new TreeMultiDict<number, string>([
  [1, 'Ein'],
  [2, 'dos'],
  [1, 'uno'],
  [2, 'Zwei'],
  [2, 'duo'],

const oneInDifferentLanguages = [...d.getValues(1)];

for (const word of oneInDifferentLanguages) {
  console.log(`The number 1 can be translated as '${word}'`);

const [added, threeCursor] = d.emplace(3, 'tres')

if (d.hasKey(3)) {
  console.log(`The dictionary now has 3 in its keys.`);
} else {
  console.log(`The dictionary does not contain 3.`);

console.log(`The dictionary now has ${d.size} elements.`)


The output of the above program:

The number 1 can be translated as as 'uno'
The number 1 can be translated as as 'Ein'
The dictionary now has 3 in its keys.
The dictionary now has 6 elements.


The sources in this library target a relatively new ECMAScript version, so that you are able to choose how much backwards-compatible the generated JavaScript should be. You are expected to use this library with a bundler such as Webpack or Rollup. Recent versions of NodeJS should also work without any bundler.

There is experimental support for tree shaking, which will result in much smaller JavaScript bundles. If you encounter an issue with this, please take your time to report it.


All collections are documented using TypeDoc, and the latest documentation is available here.

If you've found a mistake in the documentation or something is not quite clear, don't hesitate to open an issue.


Found an issue? A certain mistake? Need a certain kind of collection? File an issue or send me a pull request.


Thanks to Wolfgang De Meuter's introductory course to algorithms and data structures for teaching many of the concepts that are used in this library.

Many thanks to @w8r for providing a reference implementation of the AVL-tree data structure.


The MIT License

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  • samvv