# Instance Methods

It's

`map`

, the most important method in programming! Exactly the same as ES6 `map`

, but lazy.Usage

Type Definition

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.Usage

Type Definition

// 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.Usage

Type Definition

const sequence: Seq<[string, number]> = Seq.fromArray(["a", 1, "b", 2])

type pairwise = () => Seq<[T, T]>

Ask whether a sequence is empty.

Usage

Type Definition

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.Usage

Type Definition

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.Usage

Type Definition

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.

Usage

Type Definition

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.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.Usage

Type Definition

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.Usage

Type Definition

// 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.

Usage

Type Definition

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.

Usage

Type Definition

// 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>

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

Usage

Type Definition

// 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>

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

`uniq`

method.Usage

Type Definition

// Builds: 1 -> 2 -> 3 -> 4

const 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.Usage

Type Definition

// 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`

.Usage

Type Definition

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.

Usage

Type Definition

const doesItInclude = Seq.infinite().includes(10)

type includes = (value: T) => boolean

Lazily searches for a value that matches the predicate.

Usage

Type Definition

// Returns 11

const 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.Usage

Type Definition

// 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

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.Usage

Type Definition

// 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

Usage

Type Definition

// 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

Usage

Type Definition

// 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

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

Usage

Type Definition

// Grabs 0 -> 1 -> 2 -> 3 -> 4

const 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.

Usage

Type Definition

// Gives 0 -> 1 -> 2 -> 3 -> 4

const 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.

Usage

Type Definition

// Gives 5 -> 6 -> 7 -> 8 -> 9

const 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.

Usage

Type Definition

// 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>

Returns the

`nth`

item. Items are 1-indexed.Usage

Type Definition

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

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

Returns the

`index`

item. Items are 0-indexed.Usage

Type Definition

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

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

Gets the first value in the sequence.

Usage

Type Definition

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.

Usage

Type Definition

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.

Usage

Type Definition

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>

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.

Usage

Type Definition

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.

Usage

Type Definition

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>

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

Usage

Type Definition

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

type toArray = () => T[]

Usage

Type Definition

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

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

Usage

Type Definition

// Returns 0 + 1 + 2 + 3 + 4 = 10

const 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.

Usage

// 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)

</div>

<div data-gb-custom-block data-tag="tab" data-title='Type Definition'>

```typescript

type sumBy = (fn: (value: T) => number) => number;

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

Usage

Type Definition

// Returns (0 + 1 + 2 + 3 + 4) / 5 = 2

const 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.

Usage

Type Definition

// Returns (0 + 1 + 2 + 3 + 4) / 5 = 2

const 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.Usage

Type Definition

// 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.

Usage

Type Definition

// 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[]>

Last modified 4mo ago