A fast immutable list with a functional API.
List is a purely functional alternative to arrays. It is an
implementation of a fast persistent sequence data structure. Compared
to JavaScript's Array List has two major benefits.
Array
and a lot of other functions that'll save the day once you need
them.R.curry and where equality comparisons are
done using R.equals.import * as L from "list" in itself
adds zero bytes to your bundle when using Webpack. Using a function
adds only that function and the very small (<1KB) core of the
library. You only pay in size for the functions that you actually
use.for..of loops, works with
destructuring, and can be passed to any function expecting an
iterable. See more.This section explains how to get started using List. First you'll have to install the library.
npm install list
Then you can import it.
// As an ES module
import * as L from "list";
// Or with require
const L = require("list");
Then you can begin using List instead of arrays and enjoy immutability the performance benefits.
As a replacement for array literals List offers the function list
for constructing lists:
// An array literal
const myArray = [0, 1, 2, 3];
// A list "literal"
const myList = L.list(0, 1, 2, 3);
List has all the common functions that you know from native arrays and other libraries.
const myList = L.list(0, 1, 2, 3, 4, 5);
myList.length; //=> 6
L.filter(isEven, myList); //=> list(0, 2, 4)
L.map(n => n * n, myList); //=> list(0, 1, 4, 9, 16, 25)
L.reduce((sum, n) => sum + n, 0, myList); //=> 15
L.slice(2, 5, myList); //=> list(2, 3, 4)
L.concat(myList, L.list(6, 7, 8)); //=> list(0, 1, 2, 3, 4, 5, 6, 7, 8);
You'll probably also end up needing to convert between arrays and
List. You can do that with the functions fromArray and toArray.
L.toArray(L.list("foo", "bar")); //=> ["foo", "bar"];
L.fromArray(["foo", "bar"]); //=> L.list("foo", "bar");
List offers a wealth of other useful and high-performing functions. You can see them all in the API documentation
List implements the JavaScript iterable protocol. This means that lists can be used with array destructuring just like normal arrays.
const myList = L.list("first", "second", "third", "fourth");
const [first, second] = myList;
first; //=> "first"
second; //=> "second"
Lists can also be used in for..of loops.
for (const element of myList) {
console.log(element);
}
// logs: first, second, third, fourth
And they can be passed to any function that takes an iterable as its
argument. As an example a list can be converted into a native
Set.
const mySet = new Set(myList);
mySet.has("third"); //=> true
This works because the Set constructor accepts any iterable as
argument.
Lists also work with spread syntax. For instannce, you can call a function like this.
console.log(...list("hello", "there", "i'm", "logging", "elements"));
And each element of the list will be passed as an argument to console.log.
The iterable protocol allows for some very convenient patterns and means that lists can integrate nicely with JavaScript syntax. But, here are two anti-patterns that you should be aware of.
for..of loops. Functions like map and
foldl are often a better choice. If you want to perform
a side-effect for each element in a list you should probably use
forEach.const [a, b, ...cs] = myList; // Don't do this
const [[a, b], cs] = splitAt(2, myList); // Do this
splitAt function which splits and
creates the list cs very efficiently in O(log(n)) time.List is designed to work seamlessly together with Ramda. Ramda offers a large number of useful functions for working with arrays. List implements the same methods on its immutable data structure. This means that Ramda users can keep using the API they're familiar with.
Additionally, List offers an entry point where all functions are
curried using Ramda's R.curry and where all equality comparisons are
done using R.equals.
import * as L from "list/ramda";
const indexOfFoo1 = indexOf({ foo: 1 });
indexOfFoo1(list({ foo: 0 }, { foo: 1 }, { foo: 2 })); //=> 1
In the example above indexOf is curried and it uses R.equals to
find an element equivalent to { foo: 1 }.
Since List implements Ramda's array API it is very easy to convert code from using arrays to using immutable lists. As an example, consider the code below.
import * as R from "ramda";
R.pipe(R.filter(n => n % 2 === 0), R.map(R.multiply(3)), R.reduce(R.add, 0))(
array
);
It can be converted to code using List as follows.
import * as R from "ramda";
import * as L from "list";
R.pipe(L.filter(n => n % 2 === 0), L.map(R.multiply(3)), L.reduce(R.add, 0))(
list
);
For each function operating on arrays, the R is simply changed to an
L. This works because List exports functions that have the same name
and behavior as Ramdas functions.
The goal is to implement the entirety of Ramda's array functions for List. The list below keeps track of how many of Ramda functions that are missing and of how many that are already implemented. Currently 45 out of 75 functions have been implemented.
Implemented: adjust, all, any, append, chain, concat,
contains, drop, dropLast, dropWhile, filter, find,
findIndex, head, flatten, indexOf, init, insert,
insertAll, last, length, join, map, none, nth, pair,
partition, pluck, prepend, range, reduce, reduceRight,
reject, remove, reverse, repeat, slice, splitAt, take,
takeWhile, tail, takeLast, times, update,
zip, zipWith.
Not implemented: aperture, dropLastWhile, dropRepeats,
dropRepeatsWith, endsWith, findLast, findLastIndex,
groupWith, indexBy, intersperse, lastIndexOf, mapAccum,
mapAccumRight, reduceWhile, scan, sequence, sort,
splitEvery, splitWhen, startsWith, takeLastWhile, transpose,
traverse, unfold, uniq, uniqBy, uniqWith,
unnest without, xprod.
List currently implements the following Fantasy Land and Static Land specifications: Setoid, semigroup, monoid, foldable, functor, apply, applicative, chain, monad.
The following specifications have not been implemented yet: Traversable.
Since methods hinder tree-shaking the Fantasy Land methods are not included by default. In order to get them you must import it likes this:
import "list/fantasy-land";
The API is organized into three parts.
listCreates a list based on the arguments given.
Complexity: O(n)
Example
const l = list(1, 2, 3, 4); // creates a list of four elements
const l2 = list("foo"); // creates a singleton
emptyReturns an empty list.
Complexity: O(1)
Example
const emptyList = empty(); //=> list()
ofTakes a single arguments and returns a singleton list that contains it.
Complexity: O(1)
Example
of("foo"); //=> list("foo")
pairTakes two arguments and returns a list that contains them.
Complexity: O(1)
Example
pair("foo", "bar"); //=> list("foo", "bar")
fromArrayConverts an array into a list.
Complexity: O(n)
Example
fromArray([0, 1, 2, 3, 4]); //=> list(0, 1, 2, 3, 4)
rangeReturns a list of numbers between an inclusive lower bound and an exclusive upper bound.
Complexity: O(n)
Example
range(3, 8); //=> list(3, 4, 5, 6, 7)
repeatReturns a list of a given length that contains the specified value in all positions.
Complexity: O(n)
Example
repeat(1, 7); //=> list(1, 1, 1, 1, 1, 1, 1)
repeat("foo", 3); //=> list("foo", "foo", "foo")
timesReturns a list of given length that contains the value of the given function called with current index.
Complexity: O(n)
Example
const twoFirsOdds = times(i => i * 2 + 1, 2);
const dots = times(() => {
const x = Math.random() * width;
const y = Math.random() * height;
return { x, y };
}, 50);
concatConcatenates two lists.
Complexity: O(log(n))
Example
concat(list(0, 1, 2), list(3, 4)); //=> list(0, 1, 2, 3, 4)
flattenFlattens a list of lists into a list. Note that this function does not flatten recursively. It removes one level of nesting only.
Complexity: O(n * log(m)) where n is the length of the outer
list and m the length of the inner lists.
Example
const nested = list(list(0, 1, 2, 3), list(4), empty(), list(5, 6));
flatten(nested); //=> list(0, 1, 2, 3, 4, 5, 6)
prependPrepends an element to the front of a list and returns the new list.
Complexity: O(log(n)), practically constant
Example
const newList = prepend(0, list(1, 2, 3)); //=> list(0, 1, 2, 3)
appendAppends an element to the end of a list and returns the new list.
Complexity: O(log(n)), practically constant
Example
const newList = append(3, list(0, 1, 2)); //=> list(0, 1, 2, 3)
mapApplies a function to each element in the given list and returns a new list of the values that the function return.
Complexity: O(n)
Example
map(n => n * n, list(0, 1, 2, 3, 4)); //=> list(0, 1, 4, 9, 16)
pluckExtracts the specified property from each object in the list.
Example
const l = list(
{ foo: 0, bar: "a" },
{ foo: 1, bar: "b" },
{ foo: 2, bar: "c" }
);
pluck("foo", l); //=> list(0, 1, 2)
updateReturns a list that has the entry specified by the index replaced with the given value.
If the index is out of bounds the given list is returned unchanged.
Complexity: O(log(n))
Example
update(2, "X", list("a", "b", "c", "d", "e")); //=> list("a", "b", "X", "d", "e")
adjustReturns a list that has the entry specified by the index replaced with the value returned by applying the function to the value.
If the index is out of bounds the given list is returned unchanged.
Complexity: O(log(n))
Example
adjust(2, inc, list(0, 1, 2, 3, 4, 5)); //=> list(0, 1, 3, 3, 4, 5)
sliceReturns a slice of a list. Elements are removed from the beginning and end. Both the indices can be negative in which case they will count from the right end of the list.
Complexity: O(log(n))
Example
const l = list(0, 1, 2, 3, 4, 5);
slice(1, 4, l); //=> list(1, 2, 3)
slice(2, -2, l); //=> list(2, 3)
takeTakes the first n elements from a list and returns them in a new list.
Complexity: O(log(n))
Example
take(3, list(0, 1, 2, 3, 4, 5)); //=> list(0, 1, 2)
takeWhileTakes the first elements in the list for which the predicate returns
true.
Complexity: O(k + log(n)) where k is the number of elements
satisfying the predicate.
Example
takeWhile(n => n < 4, list(0, 1, 2, 3, 4, 5, 6)); //=> list(0, 1, 2, 3)
takeLastTakes the last n elements from a list and returns them in a new
list.
Complexity: O(log(n))
Example
takeLast(3, list(0, 1, 2, 3, 4, 5)); //=> list(3, 4, 5)
splitAtSplits a list at the given index and return the two sides in a pair. The left side will contain all elements before but not including the element at the given index. The right side contains the element at the index and all elements after it.
Complexity: O(log(n))
Example
const l = list(0, 1, 2, 3, 4, 5, 6, 7, 8);
splitAt(4, l); //=> [list(0, 1, 2, 3), list(4, 5, 6, 7, 8)]
removeTakes an index, a number of elements to remove and a list. Returns a new list with the given amount of elements removed from the specified index.
Complexity: O(log(n))
Example
const l = list(0, 1, 2, 3, 4, 5, 6, 7, 8);
remove(4, 3, l); //=> list(0, 1, 2, 3, 7, 8)
remove(2, 5, l); //=> list(0, 1, 7, 8)
dropReturns a new list without the first n elements.
Complexity: O(log(n))
Example
drop(2, list(0, 1, 2, 3, 4, 5)); //=> list(2, 3, 4, 5)
dropWhileRemoves the first elements in the list for which the predicate returns
true.
Complexity: O(k + log(n)) where k is the number of elements
satisfying the predicate.
Example
dropWhile(n => n < 4, list(0, 1, 2, 3, 4, 5, 6)); //=> list(4, 5, 6)
dropLastReturns a new list without the last n elements.
Complexity: O(log(n))
Example
dropLast(2, list(0, 1, 2, 3, 4, 5)); //=> list(0, 1, 2, 3)
tailReturns a new list with the first element removed.
Complexity: O(1)
Example
tail(list(0, 1, 2, 3)); //=> list(1, 2, 3)
popReturns a new list with the last element removed.
Aliases: init
Complexity: O(1)
Example
pop(list(0, 1, 2, 3)); //=> list(0, 1, 2)
filterReturns a new list that only contains the elements of the original
list for which the predicate returns true.
Complexity: O(n)
Example
filter(isEven, list(0, 1, 2, 3, 4, 5, 6)); //=> list(0, 2, 4, 6)
rejectReturns a new list that only contains the elements of the original
list for which the predicate returns false.
Complexity: O(n)
Example
reject(isEven, list(0, 1, 2, 3, 4, 5, 6)); //=> list(1, 3, 5)
reverseReverses a list.
Complexity: O(n)
Example
reverse(list(0, 1, 2, 3, 4, 5)); //=> list(5, 4, 3, 2, 1, 0)
apApplies a list of functions to a list of values.
Example
ap(list((n: number) => n + 2, n => 2 * n, n => n * n), list(1, 2, 3)); //=> list(3, 4, 5, 2, 4, 6, 1, 4, 9)
chainMaps a function over a list and concatenates all the resulting lists together.
Also known as flatMap.
Example
chain(n => list(n, 2 * n, n * n), list(1, 2, 3)); //=> list(1, 2, 1, 2, 4, 4, 3, 6, 9)
partitionSplits the list into two lists. One list that contains all the values
for which the predicate returns true and one containing the values for
which it returns false.
Complexity: O(n)
Example
partition(isEven, list(0, 1, 2, 3, 4, 5)); //=> list(list(0, 2, 4), list(1, 3, 5))
insertInserts the given element at the given index in the list.
Complexity: O(log(n))
Example
insert(2, "c", list("a", "b", "d", "e")); //=> list("a", "b", "c", "d", "e")
insertAllInserts the given list of elements at the given index in the list.
Complexity: O(log(n))
Example
insertAll(2, list("c", "d"), list("a", "b", "e", "f")); //=> list("a", "b", "c", "d", "e", "f")
zipWithThis is like mapping over two lists at the same time. The two lists are iterated over in parallel and each pair of elements is passed to the function. The returned values are assembled into a new list.
The shortest list determine the size of the result.
Complexity: O(log(n)) where n is the length of the smallest list.
Example
const names = list("Turing", "Curry");
const years = list(1912, 1900);
zipWith((name, year) => ({ name, year }), names, years);
//=> list({ name: "Turing", year: 1912 }, { name: "Curry", year: 1900 });
zipIterate over two lists in parallel and collect the pairs.
Complexity: O(log(n)) where n is the length of the smallest list.
Example
const names = list("a", "b", "c", "d", "e");
const years = list(0, 1, 2, 3, 4, 5, 6);
//=> list(["a", 0], ["b", 1], ["c", 2], ["d", 3], ["e", 4]);
isListReturns true if the given argument is a list.
Complexity: O(1)
Example
isList([0, 1, 2]); //=> false
isList("string"); //=> false
isList({ foo: 0, bar: 1 }); //=> false
isList(list(0, 1, 2)); //=> true
equalsReturns true if the two lists are equivalent.
Complexity: O(n)
Example
equals(list(0, 1, 2, 3), list(0, 1, 2, 3)); //=> true
equals(list("a", "b", "c"), list("a", "z", "c")); //=> false
toArrayConverts a list into an array.
Complexity: O(n)
Example
toArray(list(0, 1, 2, 3, 4)); //=> [0, 1, 2, 3, 4]
nthGets the nth element of the list. If n is out of bounds
undefined is returned.
Complexity: O(log(n)), practically constant
Example
const l = list(0, 1, 2, 3, 4);
nth(2, l); //=> 2
lengthReturns the length of a list. I.e. the number of elements that it contains.
Complexity: O(1)
Example
length(list(0, 1, 2, 3)); //=> 4
firstReturns the first element of the list. If the list is empty the
function returns undefined.
Aliases: head
Complexity: O(1)
Example
first(list(0, 1, 2, 3)); //=> 0
first(list()); //=> undefined
lastReturns the last element of the list. If the list is empty the
function returns undefined.
Complexity: O(1)
Example
last(list(0, 1, 2, 3)); //=> 3
last(list()); //=> undefined
foldlFolds a function over a list. Left-associative.
Aliases: reduce
Complexity: O(n)
Example
foldl((n, m) => n - m, 1, list(2, 3, 4, 5));
1 - 2 - 3 - 4 - 5; //=> -13
foldrFolds a function over a list. Right-associative.
Aliases: reduceRight
Complexity: O(n)
Example
foldr((n, m) => n - m, 5, list(1, 2, 3, 4));
1 - (2 - (3 - (4 - 5))); //=> 3
forEachInvokes a given callback for each element in the list from left to
right. Returns undefined.
This function is very similar to map. It should be used instead of
map when the mapping function has side-effects. Whereas map
constructs a new list forEach merely returns undefined. This makes
forEach faster when the new list is unneeded.
Complexity: O(n)
Example
const l = list(0, 1, 2);
forEach(element => console.log(element));
//=> 0
//=> 1
//=> 2
everyReturns true if and only if the predicate function returns true
for all elements in the given list.
Aliases: all
Complexity: O(n)
Example
const isEven = n => n % 2 === 0;
every(isEven, empty()); //=> true
every(isEven, list(2, 4, 6, 8)); //=> true
every(isEven, list(2, 3, 4, 6, 7, 8)); //=> false
every(isEven, list(1, 3, 5, 7)); //=> false
someReturns true if and only if there exists an element in the list for
which the predicate returns true.
Aliases: any
Complexity: O(n)
Example
const isEven = n => n % 2 === 0;
some(isEven, empty()); //=> false
some(isEven, list(2, 4, 6, 8)); //=> true
some(isEven, list(2, 3, 4, 6, 7, 8)); //=> true
some(isEven, list(1, 3, 5, 7)); //=> false
indexOfReturns the index of the first element in the list that is equal to
the given element. If no such element is found the function returns
-1.
Complexity: O(n)
Example
const l = list(12, 4, 2, 89, 6, 18, 7);
indexOf(12, l); //=> 0
indexOf(89, l); //=> 3
indexOf(10, l); //=> -1
findReturns the first element for which the predicate returns true. If
no such element is found the function returns undefined.
Complexity: O(n)
Example
find(isEven, list(1, 3, 5, 6, 7, 9, 10)); //=> 6
find(isEven, list(1, 3, 5, 7, 9)); //=> undefined
findIndexReturns the index of the first element for which the predicate returns
true. If no such element is found the function returns -1.
Complexity: O(n)
Example
findIndex(isEven, list(1, 3, 5, 6, 7, 9, 10)); //=> 3
findIndex(isEven, list(1, 3, 5, 7, 9)); //=> -1
noneReturns true if and only if the predicate function returns false
for all elements in the given list.
Complexity: O(n)
Example
const isEven = n => n % 2 === 0;
none(isEven, empty()); //=> true
none(isEven, list(2, 4, 6, 8)); //=> false
none(isEven, list(2, 3, 4, 6, 7, 8)); //=> false
none(isEven, list(1, 3, 5, 7)); //=> true
includesReturns true if the list contains the specified element. Otherwise
it returns false.
Aliases: contains
Complexity: O(n)
Example
includes(3, list(0, 1, 2, 3, 4, 5)); //=> true
includes(3, list(0, 1, 2, 4, 5)); //=> false
joinConcats the strings in a list separated by a specified separator.
Complexity: O(n)
Example
join(", ", list("one", "two", "three")); //=> "one, two, three"
The benchmarks are located in the bench directory.
FAQs
Fast purely functional immutable lists.
The npm package list receives a total of 11,006 weekly downloads. As such, list popularity was classified as popular.
We found that list demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 2 open source maintainers collaborating on the project.
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