std/collections

This module includes pure functions for specific common tasks around collection types like Array and Record. This module is heavily inspired by kotlins stdlib.

All provided functions are pure, which also means that they do not mutate your inputs, returning a new value instead.
All functions are importable on their own by referencing their snake_case named file (e.g. collections/sort_by.ts)

If you want to contribute or understand why this is done the way it is, see the contribution guide.

Usage

aggregateGroups

Applies the given aggregator to each group in the given grouping, returning the results together with the respective group keys

import { aggregateGroups } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const foodProperties = {
  "Curry": ["spicy", "vegan"],
  "Omelette": ["creamy", "vegetarian"],
};
const descriptions = aggregateGroups(
  foodProperties,
  (current, key, first, acc) => {
    if (first) {
      return `${key} is ${current}`;
    }

    return `${acc} and ${current}`;
  },
);

assertEquals(descriptions, {
  "Curry": "Curry is spicy and vegan",
  "Omelette": "Omelette is creamy and vegetarian",
});

associateBy

Transforms the given array into a Record, extracting the key of each element using the given selector. If the selector produces the same key for multiple elements, the latest one will be used (overriding the ones before it).

import { associateBy } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const users = [
  { id: "a2e", userName: "Anna" },
  { id: "5f8", userName: "Arnold" },
  { id: "d2c", userName: "Kim" },
];
const usersById = associateBy(users, (it) => it.id);

assertEquals(usersById, {
  "a2e": { id: "a2e", userName: "Anna" },
  "5f8": { id: "5f8", userName: "Arnold" },
  "d2c": { id: "d2c", userName: "Kim" },
});

associateWith

Builds a new Record using the given array as keys and choosing a value for each key using the given selector. If any of two pairs would have the same value the latest on will be used (overriding the ones before it).

import { associateWith } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const names = ["Kim", "Lara", "Jonathan"];
const namesToLength = associateWith(names, (it) => it.length);

assertEquals(namesToLength, {
  "Kim": 3,
  "Lara": 4,
  "Jonathan": 8,
});

chunk

Splits the given array into chunks of the given size and returns them.

import { chunk } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const words = [
  "lorem",
  "ipsum",
  "dolor",
  "sit",
  "amet",
  "consetetur",
  "sadipscing",
];
const chunks = chunk(words, 3);

assertEquals(
  chunks,
  [
    ["lorem", "ipsum", "dolor"],
    ["sit", "amet", "consetetur"],
    ["sadipscing"],
  ],
);

deepMerge

Merges the two given Records, recursively merging any nested Records with the second collection overriding the first in case of conflict

For arrays, maps and sets, a merging strategy can be specified to either replace values, or merge them instead. Use includeNonEnumerable option to include non enumerable properties too.

import { deepMerge } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const a = { foo: true };
const b = { foo: { bar: true } };

assertEquals(deepMerge(a, b), { foo: { bar: true } });

distinctBy

Returns all elements in the given array that produce a distinct value using the given selector, preserving order by first occurrence.

import { distinctBy } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const names = ["Anna", "Kim", "Arnold", "Kate"];
const exampleNamesByFirstLetter = distinctBy(names, (it) => it.charAt(0));

assertEquals(exampleNamesByFirstLetter, ["Anna", "Kim"]);

distinct

Returns all distinct elements in the given array, preserving order by first occurrence.

import { distinct } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [3, 2, 5, 2, 5];
const distinctNumbers = distinct(numbers);

assertEquals(distinctNumbers, [3, 2, 5]);

dropWhile

Returns a new array that drops all elements in the given collection until the first element that does not match the given predicate.

import { dropWhile } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [3, 2, 5, 2, 5];
const dropWhileNumbers = dropWhile(numbers, (i) => i !== 2);

assertEquals(dropWhileNumbers, [2, 5, 2, 5]);

dropLastWhile

Returns a new array that drops all elements in the given collection until the last element that does not match the given predicate

Example:

import { dropLastWhile } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [22, 30, 44];

const notFortyFour = dropLastWhile(numbers, (i) => i != 44);

assertEquals(
  notFortyFour,
  [22, 30],
);

filterEntries

Returns a new record with all entries of the given record except the ones that do not match the given predicate.

import { filterEntries } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const menu = {
  "Salad": 11,
  "Soup": 8,
  "Pasta": 13,
};
const myOptions = filterEntries(
  menu,
  ([item, price]) => item !== "Pasta" && price < 10,
);

assertEquals(
  myOptions,
  {
    "Soup": 8,
  },
);

filterKeys

Returns a new record with all entries of the given record except the ones that have a key that does not match the given predicate.

import { filterKeys } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const menu = {
  "Salad": 11,
  "Soup": 8,
  "Pasta": 13,
};
const menuWithoutSalad = filterKeys(menu, (it) => it !== "Salad");

assertEquals(
  menuWithoutSalad,
  {
    "Soup": 8,
    "Pasta": 13,
  },
);

filterValues

Returns a new record with all entries of the given record except the ones that have a value that does not match the given predicate.

import { filterValues } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = {
  "Arnold": 37,
  "Sarah": 7,
  "Kim": 23,
};
const adults = filterValues(people, (it) => it >= 18);

assertEquals(
  adults,
  {
    "Arnold": 37,
    "Kim": 23,
  },
);

findLastIndex

Returns the index of the last element in the given array matching the given predicate.

import { findLastIndex } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [0, 1, 2, 3, 4, 5, 6];
const lastIndexEvenNumber = findLastIndex(numbers, (it) => it % 2 === 0);

assertEquals(lastIndexEvenNumber, 6);

findLast

Returns the last element in the given array matching the given predicate.

import { findLast } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [4, 2, 7];
const lastEvenNumber = findLast(numbers, (it) => it % 2 === 0);

assertEquals(lastEvenNumber, 2);

findSingle

Returns an element if and only if that element is the only one matching the given condition. Returns undefined otherwise.

import { findSingle } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const bookings = [
  { month: "January", active: false },
  { month: "March", active: false },
  { month: "June", active: true },
];
const activeBooking = findSingle(bookings, (it) => it.active);
const inactiveBooking = findSingle(bookings, (it) => !it.active);

assertEquals(activeBooking, { month: "June", active: true });
assertEquals(inactiveBooking, undefined); // there are two applicable items

firstNotNullishOf

Applies the given selector to elements in the given array until a value is produced that is neither null nor undefined and returns that value. Returns undefined if no such value is produced

import { firstNotNullishOf } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const tables = [
  { number: 11, order: null },
  { number: 12, order: "Soup" },
  { number: 13, order: "Salad" },
];
const nextOrder = firstNotNullishOf(tables, (it) => it.order);

assertEquals(nextOrder, "Soup");

groupBy

Applies the given selector to each element in the given array, returning a Record containing the results as keys and all values that produced that key as values.

import { groupBy } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = [
  { name: "Anna" },
  { name: "Arnold" },
  { name: "Kim" },
];
const peopleByFirstLetter = groupBy(people, (it) => it.name.charAt(0));

assertEquals(
  peopleByFirstLetter,
  {
    "A": [{ name: "Anna" }, { name: "Arnold" }],
    "K": [{ name: "Kim" }],
  },
);

includesValue

If the given value is part of the given object it returns true, otherwise it returns false. Doesn't work with non-primitive values: includesValue({x: {}}, {}) returns false.

import { includesValue } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const input = {
  first: 33,
  second: 34,
};

assertEquals(includesValue(input, 34), true);

intersect

Returns all distinct elements that appear at least once in each of the given arrays.

import { intersect } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const lisaInterests = ["Cooking", "Music", "Hiking"];
const kimInterests = ["Music", "Tennis", "Cooking"];
const commonInterests = intersect(lisaInterests, kimInterests);

assertEquals(commonInterests, ["Cooking", "Music"]);

joinToString

Transforms the elements in the given array to strings using the given selector. Joins the produced strings into one using the given separator and applying the given prefix and suffix to the whole string afterwards. If the array could be huge, you can specify a non-negative value of limit, in which case only the first limit elements will be appended, followed by the truncated string. Returns the resulting string.

import { joinToString } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const users = [
  { name: "Kim" },
  { name: "Anna" },
  { name: "Tim" },
];

const message = joinToString(users, (it) => it.name, {
  suffix: " are winners",
  prefix: "result: ",
  separator: " and ",
  limit: 1,
  truncated: "others",
});

assertEquals(message, "result: Kim and others are winners");

mapEntries

Applies the given transformer to all entries in the given record and returns a new record containing the results.

import { mapEntries } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const usersById = {
  "a2e": { name: "Kim", age: 22 },
  "dfe": { name: "Anna", age: 31 },
  "34b": { name: "Tim", age: 58 },
};
const agesByNames = mapEntries(usersById, ([id, { name, age }]) => [name, age]);

assertEquals(
  agesByNames,
  {
    "Kim": 22,
    "Anna": 31,
    "Tim": 58,
  },
);

mapKeys

Applies the given transformer to all keys in the given record's entries and returns a new record containing the transformed entries.

If the transformed entries contain the same key multiple times, only the last one will appear in the returned record.

import { mapKeys } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const counts = { a: 5, b: 3, c: 8 };

assertEquals(
  mapKeys(counts, (it) => it.toUpperCase()),
  {
    A: 5,
    B: 3,
    C: 8,
  },
);

mapNotNullish

Returns a new array, containing all elements in the given array transformed using the given transformer, except the ones that were transformed to null or undefined.

import { mapNotNullish } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = [
  { middleName: null },
  { middleName: "William" },
  { middleName: undefined },
  { middleName: "Martha" },
];
const foundMiddleNames = mapNotNullish(people, (it) => it.middleName);

assertEquals(foundMiddleNames, ["William", "Martha"]);

mapValues

Applies the given transformer to all values in the given record and returns a new record containing the resulting keys associated to the last value that produced them.

import { mapValues } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const usersById = {
  "a5ec": { name: "Mischa" },
  "de4f": { name: "Kim" },
};
const namesById = mapValues(usersById, (it) => it.name);

assertEquals(
  namesById,
  {
    "a5ec": "Mischa",
    "de4f": "Kim",
  },
);

maxBy

Returns the first element that is the largest value of the given function or undefined if there are no elements.

import { maxBy } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = [
  { name: "Anna", age: 34 },
  { name: "Kim", age: 42 },
  { name: "John", age: 23 },
];

const personWithMaxAge = maxBy(people, (i) => i.age);

assertEquals(personWithMaxAge, { name: "Kim", age: 42 });

maxOf

Applies the given selector to all elements of the provided collection and returns the max value of all elements. If an empty array is provided the function will return undefined

import { maxOf } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const inventory = [
  { name: "mustard", count: 2 },
  { name: "soy", count: 4 },
  { name: "tomato", count: 32 },
];

const maxCount = maxOf(inventory, (i) => i.count);

assertEquals(maxCount, 32);

maxWith

Returns the first element having the largest value according to the provided comparator or undefined if there are no elements

import { maxWith } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = ["Kim", "Anna", "John", "Arthur"];
const largestName = maxWith(people, (a, b) => a.length - b.length);

assertEquals(largestName, "Arthur");

minBy

Returns the first element that is the smallest value of the given function or undefined if there are no elements

import { minBy } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = [
  { name: "Anna", age: 34 },
  { name: "Kim", age: 42 },
  { name: "John", age: 23 },
];

const personWithMinAge = minBy(people, (i) => i.age);

assertEquals(personWithMinAge, { name: "John", age: 23 });

minOf

Applies the given selector to all elements of the given collection and returns the min value of all elements. If an empty array is provided the function will return undefined

import { minOf } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const inventory = [
  { name: "mustard", count: 2 },
  { name: "soy", count: 4 },
  { name: "tomato", count: 32 },
];
const minCount = minOf(inventory, (i) => i.count);

assertEquals(minCount, 2);

minWith

Returns the first element having the smallest value according to the provided comparator or undefined if there are no elements

import { minWith } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = ["Kim", "Anna", "John"];
const smallestName = minWith(people, (a, b) => a.length - b.length);

assertEquals(smallestName, "Kim");

partition

Returns a tuple of two arrays with the first one containing all elements in the given array that match the given predicate and the second one containing all that do not.

import { partition } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [5, 6, 7, 8, 9];
const [even, odd] = partition(numbers, (it) => it % 2 == 0);

assertEquals(even, [6, 8]);
assertEquals(odd, [5, 7, 9]);

permutations

Builds all possible orders of all elements in the given array Ignores equality of elements, meaning this will always return the same number of permutations for a given length of input.

import { permutations } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [1, 2];
const windows = permutations(numbers);

assertEquals(
  windows,
  [
    [1, 2],
    [2, 1],
  ],
);

reduceGroups

Applies the given reducer to each group in the given Grouping, returning the results together with the respective group keys

import { reduceGroups } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const votes = {
  "Woody": [2, 3, 1, 4],
  "Buzz": [5, 9],
};

const totalVotes = reduceGroups(votes, (sum, it) => sum + it, 0);

assertEquals(totalVotes, {
  "Woody": 10,
  "Buzz": 14,
});

runningReduce

Calls the given reducer on each element of the given collection, passing it's result as the accumulator to the next respective call, starting with the given initialValue. Returns all intermediate accumulator results.

Example:

import { runningReduce } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [1, 2, 3, 4, 5];
const sumSteps = runningReduce(numbers, (sum, current) => sum + current, 0);

assertEquals(sumSteps, [1, 3, 6, 10, 15]);

sumOf

Applies the given selector to all elements in the given collection and calculates the sum of the results.

import { sumOf } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = [
  { name: "Anna", age: 34 },
  { name: "Kim", age: 42 },
  { name: "John", age: 23 },
];
const totalAge = sumOf(people, (i) => i.age);

assertEquals(totalAge, 99);

sample

Returns a random element from the given array

import { sample } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assert } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [1, 2, 3, 4];
const random = sample(numbers);

assert(numbers.includes(random as number));

slidingWindows

Generates sliding views of the given array of the given size and returns a new array containing all of them.

If step is set, each window will start that many elements after the last window's start. (Default: 1)

If partial is set, windows will be generated for the last elements of the collection, resulting in some undefined values if size is greater than 1. (Default: false)

import { slidingWindows } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";
const numbers = [1, 2, 3, 4, 5];

const windows = slidingWindows(numbers, 3);
assertEquals(windows, [
  [1, 2, 3],
  [2, 3, 4],
  [3, 4, 5],
]);

const windowsWithStep = slidingWindows(numbers, 3, { step: 2 });
assertEquals(windowsWithStep, [
  [1, 2, 3],
  [3, 4, 5],
]);

const windowsWithPartial = slidingWindows(numbers, 3, { partial: true });
assertEquals(windowsWithPartial, [
  [1, 2, 3],
  [2, 3, 4],
  [3, 4, 5],
  [4, 5],
  [5],
]);

sortBy

Returns all elements in the given collection, sorted by their result using the given selector. The selector function is called only once for each element.

import { sortBy } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const people = [
  { name: "Anna", age: 34 },
  { name: "Kim", age: 42 },
  { name: "John", age: 23 },
];
const sortedByAge = sortBy(people, (it) => it.age);

assertEquals(sortedByAge, [
  { name: "John", age: 23 },
  { name: "Anna", age: 34 },
  { name: "Kim", age: 42 },
]);

takeLastWhile

Returns all elements in the given array after the last element that does not match the given predicate.

Example:

import { takeLastWhile } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const arr = [1, 2, 3, 4, 5, 6];

assertEquals(
  takeLastWhile(arr, (i) => i > 4),
  [5, 6],
);

takeWhile

Returns all elements in the given collection until the first element that does not match the given predicate.

import { takeWhile } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const arr = [1, 2, 3, 4, 5, 6];

assertEquals(
  takeWhile(arr, (i) => i !== 4),
  [1, 2, 3],
);

union

Returns all distinct elements that appear in any of the given arrays

import { union } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const soupIngredients = ["Pepper", "Carrots", "Leek"];
const saladIngredients = ["Carrots", "Radicchio", "Pepper"];
const shoppingList = union(soupIngredients, saladIngredients);

assertEquals(shoppingList, ["Pepper", "Carrots", "Leek", "Radicchio"]);

unzip

Builds two separate arrays from the given array of 2-tuples, with the first returned array holding all first tuple elements and the second one holding all the second elements

import { unzip } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const parents = [
  ["Maria", "Jeff"],
  ["Anna", "Kim"],
  ["John", "Leroy"],
] as [string, string][];

const [moms, dads] = unzip(parents);

assertEquals(moms, ["Maria", "Anna", "John"]);
assertEquals(dads, ["Jeff", "Kim", "Leroy"]);

withoutAll

Returns an array excluding all given values

import { withoutAll } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const withoutList = withoutAll([2, 1, 2, 3], [1, 2]);

assertEquals(withoutList, [3]);

zip

Builds N-tuples of elements from the given N arrays with matching indices, stopping when the smallest array's end is reached

import { zip } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [1, 2, 3, 4];
const letters = ["a", "b", "c", "d"];
const pairs = zip(numbers, letters);

assertEquals(
  pairs,
  [
    [1, "a"],
    [2, "b"],
    [3, "c"],
    [4, "d"],
  ],
);

DeepMerge	Merge deeply two objects
DeepMergeOptions	Deep merge options
JoinToStringOptions	Options for joinToString
MergingStrategy	Merging strategy

Functions

Type Aliases

std/collections

Usage

aggregateGroups

associateBy

associateWith

chunk

deepMerge

distinctBy

distinct

dropWhile

dropLastWhile

filterEntries

filterKeys

filterValues

findLastIndex

findLast

findSingle

firstNotNullishOf

groupBy

includesValue

intersect

joinToString

mapEntries

mapKeys

mapNotNullish

mapValues

maxBy

maxOf

maxWith

minBy

minOf

minWith

partition

permutations

reduceGroups

runningReduce

sumOf

sample

slidingWindows

sortBy

takeLastWhile

takeWhile

union

unzip

withoutAll

zip