It's map
, the most important method in programming! Exactly the same as ES6 map
, but lazy.
const sequence: Seq<string> = Seq.infinite().map(num => num.toString());
type map = <U>(fn: (value: T, index: number) => U) => Seq<U>;
window
takes a sequence and groups it into "windows" of a certain length. This works well with infinite sequences where you want to process some number of values at a time.
// Grab numbers in groups of 10.const sequence: Seq<number[]> = Seq.infinite().window(10);
By default, only triggers chained responses when the window fills, guaranteeing the window is the exact size expect. Set allowPartialWindow
to false
to allow the trailing edge of a sequence to not be divisible by the window size.
// Gives: [0, 1, 2] -> [3, 4, 5] -> [6, 7, 8] -> [9, 10]const sequence: Seq<number> = Seq.range(0, 10).window(3);
type window = (size: number, allowPartialWindow = true) => Seq<T[]>;
Works like window
, makes the window size 2. Groups a sequence as alternating pairs. Useful for processing data which alternates Map keys and values.
const sequence: Seq<[string, number]> = Seq.fromArray(["a", 1, "b", 2]);
type pairwise = () => Seq<[T, T]>;
Ask whether a sequence is empty.
const anythingInThere: boolean = Seq.empty().isEmpty();
type isEmpty = () => boolean;
tap
lets you run side-effect generating functions on a sequence. Allows you to "tap in to" a data flow. Very useful for logging and debugging what values are flowing through the chain at a given location.
const sequence: Seq<number> = Seq.infinite().tap(num => console.log(num));
type tap = (fn: (value: T, index: number) => void) => Seq<T>;
log
provides the most common use-case for `tap. Add this to a sequence chain to log each value that passes through it.
const sequence: Seq<number> = Seq.infinite().log();
type log = () => Seq<T>;
Given a sequence where each item in an array of items, flatten all those arrays into a single flat sequence of values.
Works just like Array.prototype.flat
. See more here.​
type Person = { name: string; friends: Person[] };​const sequence: Seq<Friend> = Seq.fromArray([person1, person2]).map(person => person.friends).flat();
type flat = <U>(this: Seq<U[]>) => Seq<U>;
flatMap
is used when mapping a list to each items related items. For example, if you wanted to map from a list of people to each persons list of friends. Despite each mapping function returning an array, the final output is a flatten array of all the results concattenated.
Works just like Array.prototype.flatMap
. See more here.​
Similar to [].map().flat()
, but in leisure
the item mappings won't execute until enough of the resulting values have been realized to trigger each map.
type Person = { name: string; friends: Person[] };​const sequence: Seq<Friend> = Seq.fromArray([person1, person2]).flatMap(person => person.friends);
type flatMap = <U>(fn: (value: T, index: number) => U[]) => Seq<U>;
Runs a predicate function on each item in a sequence to produce a new sequence where only the values which responded with true
remain.
Exactly the same as Array.prototype.filter
, but lazy. See more here.
// Create a sequence of only even numbers.const sequence: Seq<number> = Seq.infinite().filter(num => num % 2 === 0);
type filter = (fn: (value: T, index: number) => unknown) => Seq<T>;
Combines the current sequence with 1 or more additional sequences.
Exactly the same as Array.prototype.concat
, but lazy. See more here.
const sequence: Seq<number> = Seq.fromArray([0, 1]).concat(Seq.fromArray([2, 3]),Seq.fromArray([4, 5]));
type concat = (...tail: Array<Seq<T>>) => Seq<T>;
Takes 1 or more sequences and creates a new sequence built by pulling the next value from each of the sequences in order.
// Builds: a -> 1 -> b -> 2 -> c -> 3const sequence: Seq<string | number> = Seq.fromArray(["a","b","c"]).interleave(Seq.range(1, 3));
type interleave = (...tail: Array<Seq<T>>) => Seq<T>;
Given a sequence, place a value between each value of the original sequence. Useful for adding punctuation between strings.
// Builds: Apples -> , -> Oranges -> , -> Bananasconst sequence: Seq<string> = Seq.fromArray(["Apples","Oranges","Bananas"]).interpose(", ");​console.log(sequence.toArray().join(""));
type interpose = (separator: T) => Seq<T>;
Given a sequence, only forwards the values which have no already been seen. Very similar to lodash's uniq
method.
// Builds: 1 -> 2 -> 3 -> 4const sequence: Seq<number> = Seq.fromArray([1, 2, 3, 2, 1, 4]).distinct();
type distinct = () => Seq<T>;
Same as distinct
, but allows a function to describe on what value the sequence should be unique.
// Builds: { firstName: "A", lastName: "Z" } ->// { firstName: "B", lastName: "Y" } ->// { firstName: "C", lastName: "W" }type Person = { firstName: string; lastName: string };const sequence: Seq<Person> = Seq.fromArray([{ firstName: "A", lastName: "Z" },{ firstName: "B", lastName: "Y" },{ firstName: "A", lastName: "X" },{ firstName: "C", lastName: "W" }]).distinctBy(person => person.firstName);
type distinctBy = <U>(fn: (value: T) => U) => Seq<T>;
Given a sequence, splits the values into two separate sequences. One represents the values where the partition function is true
and the other for false
.
const [isEven, isOdd] = Seq.infinite().partitionBy(num => num % 2 === 0);
type partition = (fn: (value: T, index: number) => unknown) => [Seq<T>, Seq<T>];
Lazily checks if the sequence includes a value.
Exactly the same as Array.prototype.includes
, but lazy. See more here.
const doesItInclude = Seq.infinite().includes(10);
type includes = (value: T) => boolean;
Lazily searches for a value that matches the predicate.
Exactly the same as Array.prototype.find
, but lazy. See more here.
// Returns 11const gtTen = Seq.infinite().find(num => num > 10);
type find = (fn: (value: T, index: number) => unknown) => T | undefined;
Exactly the same as Array.prototype.reduce
. See more here. This causes a full realization of the data. Not lazy.
// Returns 0 + 1 + 2 + 3 + 4 = 10const sum = Seq.infinite().take(5).reduce((sum, num) => sum + num);
type reduce = <A>(fn: (sum: A, value: T, index: number) => A, initial: A) => A;
This method is helpful for chaining. Shocking, I know. Let's you "map" the entire sequence in a chain, rather than per-each-item. Allows adding arbitrary sequence helpers and methods to chain, even if they are written in user-land and not on the Seq
prototype.
// Same as `Seq.interpose(Seq.infinite(), Seq.infinite())`const sequence = Seq.infinite().chain(seq => seq.interpose(Seq.infinite()));
type chain = <U>(fn: (value: Seq<T>) => U) => U;
Exactly the same as Array.prototype.some
, but lazy. See more here.
// Find the first even random number.const areAnyEven = Seq.random().map(num => Math.round(num * 1000)).some(num => num % 2 === 0);
type some = (fn: (value: T, index: number) => unknown) => boolean;
Exactly the same as Array.prototype.every
, but lazy. See more here.
// Fails fast if there are negative numbersconst areAllPositive = Seq.random().map(num => Math.round(num * 1000) - 500).every(num => num > 0);
type every = (fn: (value: T, index: number) => unknown) => boolean;
Given a sequence of unknown length, create a sub sequence of just the first X number of items.
// Grabs 0 -> 1 -> 2 -> 3 -> 4const firstFive = Seq.infinite().take(5);
type take = (num: number) => Seq<T>;
Given a sequence of unknown length, create a sub sequence of as many items in a row that satisfy the predicate.
// Gives 0 -> 1 -> 2 -> 3 -> 4const lessThanFive = Seq.infinite().takeWhile(num => num < 5);
type takeWhile = (fn: (value: T, index: number) => unknown) => Seq<T>;
Given a sequence of unknown length, skips the first X number of items.
// Gives 5 -> 6 -> 7 -> 8 -> 9const secondFive = Seq.infinite().skip(5).take(5);
type skip = (num: number) => Seq<T>;
Given a sequence of unknown length, skip as many items in a row that satisfy the predicate.
// Gives 5 -> 6 -> 7 -> 8 -> 9const greaterThanFive = Seq.infinite().skipWhile(num => num < 5).take(5);
type skipWhile = (fn: (value: T, index: number) => unknown) => Seq<T>;
Returns the nth
item. Items are 1-indexed.
const thirdItem = Seq.infinite().nth(3);
type nth = (i: number) => T | undefined;
Returns the index
item. Items are 0-indexed.
const fourthItem = Seq.infinite().index(3);
type nth = (i: number) => T | undefined;
Gets the first value in the sequence.
const fifth = Seq.infinite().skip(4).first();
type first = () => T | undefined;
Lazily combines a second sequence with this current one to produce a tuple with the current step in each of the two positions. Useful for zipping a sequence of keys with a sequence of values, before converting to a Map of key to value.
const seq2 = Seq.range(0, 3);​// Gives: ["zero", 0] -> ["one", 1] -> ["two", 2] -> ["three", 3]const sequence: Seq<[string, number]> = Seq.fromArray(["zero","one","two","three"]).zip(seq2);
type zip<T2> = (seq2: Seq<T2>) => Seq<[T | undefined, T2 | undefined]>;
Takes a second sequence and lazily combines it to produce an arbitrary value by mapping the current value of the two positions through a user-supplied function. Useful for table (row/col) math.
const seq2 = Seq.repeat(2);​// Gives: 0 -> 2 -> 4 -> 6const sequence: Seq<number> = Seq.range(0, 3).zipWith(([num, multiplier]) => num * multiplier,seq2);
type zip2With = <T2, T3, T4>(fn: ([result1, result2, result3]:| [T, T2, T3]| [T, undefined, undefined]| [T, T2, undefined]| [T, undefined, T3]| [undefined, T2, undefined]| [undefined, T2, T3]| [undefined, undefined, T3],index: number) => T4,seq2: Seq<T2>,seq3: Seq<T3>) => Seq<T4>;
Takes two sequences and lazily combines them with this one to produce a 3-tuple with the current step in each of the three positions.
const seq2 = Seq.range(0, 3);const seq3 = Seq.range(3, 0);​// Gives: ["zero", 0, 3] -> ["one", 1, 2] -> ["two", 2, 1] -> ["three", 3, 0]const sequence: Seq<[string, number]> = Seq.fromArray(["zero","one","two","three"]).zip2(seq2, seq3);
type zip2 = <T2, T3>(seq2: Seq<T2>,seq3: Seq<T3>) => Seq<[T | undefined, T2 | undefined, T3 | undefined]>;
Takes two sequences and lazily combine them with this sequence to produce an arbitrary value by mapping the current value of the three positions through a user-supplied function.
const seq2 = Seq.repeat(2);const seq3 = Seq.repeat(1);​// Gives: 0 -> 2 -> 4 -> 6const sequence: Seq<number> = Seq.range(0, 3).zip2With(([num, multiplier, divisor]) => (num * multiplier) / divisor,seq2,seq3);
type zip2With = <T2, T3, T4>(fn: ([result1, result2, result3]:| [T, T2, T3]| [T, undefined, undefined]| [T, T2, undefined]| [T, undefined, T3]| [undefined, T2, undefined]| [undefined, T2, T3]| [undefined, undefined, T3],index: number) => T4,seq2: Seq<T2>,seq3: Seq<T3>) => Seq<T4>;
Converts the sequence to a real JavaScript array. Realizes the entire sequence.
const lessThanTen = Seq.infinite().take(10).toArray();
type toArray = () => T[];
Works just like Array.prototype.forEach
. See more here. Realizes the full sequence.
​
type forEach = (fn: (value: T, index: number) => void) => void;
Given a sequence of numbers, adds them all together. This realizes the entire sequence.
// Returns 0 + 1 + 2 + 3 + 4 = 10const sum = Seq.infinite().take(5).sum();
type sum = (this: Seq<number>) => number;
Given a sequence of arbitrary data, adds together the result of the mapping function. This realizes the entire sequence.
// Returns 0 + 1 + 2 + 3 + 4 = 10const sum = Seq.fromArray([{ balance: 0 },{ balance: 1 },{ balance: 2 },{ balance: 3 },{ balance: 4 }]).sumBy(user => user.balance);
{% endtab %}​{% tab title="Type Definition" %}​```typescripttype sumBy = (fn: (value: T) => number) => number;
Given a sequence of numbers, averages them all together. Tise realizes the entire sequence.
// Returns (0 + 1 + 2 + 3 + 4) / 5 = 2const sum = Seq.infinite().take(5).average();
type average = (this: Seq<number>) => number;
Given a sequence of arbitrary data, averages together the result of the mapping function. This realizes the entire sequence.
// Returns (0 + 1 + 2 + 3 + 4) / 5 = 2const sum = Seq.fromArray([{ balance: 0 },{ balance: 1 },{ balance: 2 },{ balance: 3 },{ balance: 4 }]).averageBy(user => user.balance);
type averageBy = (fn: (value: T) => number) => number;
Given a non-infinite sequence, return a Map
which counts the occurances of each unique value. This realizes the entire sequence.
// Returns a Map of numbers from 0->100 and how many times they randomly occured in this set of 500.const freq = Seq.random().map(num => Math.round(num * 100)).take(500).frequencies();
type frequencies = () => Map<T, number>;
Group a sequence by the return of a mapping function. This realizes the entire sequence.
// Random generates 1000 years between 0-2000 and// groups them by decade.const groupedByDecade = Seq.random().map(num => Math.round(num * 2000)).take(100).groupBy(year => Math.round(year / 10));
type groupBy = <U>(fn: (item: T) => U) => Map<U, T[]>;