map

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

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);

// 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[]>;

pairwise

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]>;

isEmpty

Ask whether a sequence is empty.

const anythingInThere: boolean = Seq.empty().isEmpty();

type isEmpty = () => boolean;

tap

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

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>;

flat

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

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>;

filter

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>;

concat

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>;

interleave

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 -> 3
const sequence: Seq<string | number> = Seq.fromArray([
  "a",
  "b",
  "c"
]).interleave(Seq.range(1, 3));

type interleave = (...tail: Array<Seq<T>>) => Seq<T>;

interpose

Given a sequence, place a value between each value of the original sequence. Useful for adding punctuation between strings.

// Builds: Apples -> , -> Oranges -> , -> Bananas
const sequence: Seq<string> = Seq.fromArray([
  "Apples",
  "Oranges",
  "Bananas"
]).interpose(", ");
​
console.log(sequence.toArray().join(""));

type interpose = (separator: T) => Seq<T>;

distinct

Given a sequence, only forwards the values which have no already been seen. Very similar to lodash's uniq method.

// Builds: 1 -> 2 -> 3 -> 4
const sequence: Seq<number> = Seq.fromArray([1, 2, 3, 2, 1, 4]).distinct();

type distinct = () => Seq<T>;

distinctBy

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>;

partitionBy

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>];

includes

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;

find

Lazily searches for a value that matches the predicate.

Exactly the same as Array.prototype.find, but lazy. See more here.

// Returns 11
const gtTen = Seq.infinite().find(num => num > 10);

type find = (fn: (value: T, index: number) => unknown) => T | undefined;

reduce

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 = 10
const sum = Seq.infinite()
  .take(5)
  .reduce((sum, num) => sum + num);

type reduce = <A>(fn: (sum: A, value: T, index: number) => A, initial: A) => A;

chain

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;

some

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;

every

Exactly the same as Array.prototype.every, but lazy. See more here.

// Fails fast if there are negative numbers
const areAllPositive = Seq.random()
  .map(num => Math.round(num * 1000) - 500)
  .every(num => num > 0);

type every = (fn: (value: T, index: number) => unknown) => boolean;

take

Given a sequence of unknown length, create a sub sequence of just the first X number of items.

// Grabs 0 -> 1 -> 2 -> 3 -> 4
const firstFive = Seq.infinite().take(5);

type take = (num: number) => Seq<T>;

takeWhile

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 -> 4
const lessThanFive = Seq.infinite().takeWhile(num => num < 5);

type takeWhile = (fn: (value: T, index: number) => unknown) => Seq<T>;

skip

Given a sequence of unknown length, skips the first X number of items.

// Gives 5 -> 6 -> 7 -> 8 -> 9
const secondFive = Seq.infinite()
  .skip(5)
  .take(5);

type skip = (num: number) => Seq<T>;

skipWhile

Given a sequence of unknown length, skip as many items in a row that satisfy the predicate.

// Gives 5 -> 6 -> 7 -> 8 -> 9
const greaterThanFive = Seq.infinite()
  .skipWhile(num => num < 5)
  .take(5);

type skipWhile = (fn: (value: T, index: number) => unknown) => Seq<T>;

nth

Returns the nth item. Items are 1-indexed.

const thirdItem = Seq.infinite().nth(3);

type nth = (i: number) => T | undefined;

index

Returns the index item. Items are 0-indexed.

const fourthItem = Seq.infinite().index(3);

type nth = (i: number) => T | undefined;

first

Gets the first value in the sequence.

const fifth = Seq.infinite()
  .skip(4)
  .first();

type first = () => T | undefined;

zip

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]>;

zipWith

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 -> 6
const 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>;

zip2

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]>;

zip2With

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 -> 6
const 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>;

toArray

Converts the sequence to a real JavaScript array. Realizes the entire sequence.

const lessThanTen = Seq.infinite()
  .take(10)
  .toArray();

type toArray = () => T[];

forEach

Works just like Array.prototype.forEach. See more here. Realizes the full sequence.

​

type forEach = (fn: (value: T, index: number) => void) => void;

sum

Given a sequence of numbers, adds them all together. This realizes the entire sequence.

// Returns 0 + 1 + 2 + 3 + 4 = 10
const sum = Seq.infinite()
  .take(5)
  .sum();

type sum = (this: Seq<number>) => number;

sumBy

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 = 10
const sum = Seq.fromArray([
  { balance: 0 },
  { balance: 1 },
  { balance: 2 },
  { balance: 3 },
  { balance: 4 }
]).sumBy(user => user.balance);

{% endtab %}
​
{% tab title="Type Definition" %}
​
```typescript
type sumBy = (fn: (value: T) => number) => number;

average

Given a sequence of numbers, averages them all together. Tise realizes the entire sequence.

// Returns (0 + 1 + 2 + 3 + 4) / 5 = 2
const sum = Seq.infinite()
  .take(5)
  .average();