API / Js / Array

Array

Provides bindings to JavaScript’s Array functions. These bindings are optimized for pipe-last (|>), where the array to be processed is the last parameter in the function.

Here is an example to find the sum of squares of all even numbers in an array. Without pipe last, we must call the functions in reverse order:

RES
let isEven = x => mod(x, 2) == 0
let square = x => x * x
let result = {
  open Js.Array
  reduce(\"+", 0, map(square, filter(isEven, [5, 2, 3, 4, 1])))
}

With pipe last, we call the functions in the “natural” order:

RES
let isEven = x => mod(x, 2) == 0
let square = x => x * x
let result = {
  open Js.Array
  [5, 2, 3, 4, 1] |> filter(isEven) |> map(square) |> reduce(\"+", 0)
}

t

RES
type t<'a> = array<'a>

The type used to describe a JavaScript array.

array_like

RES
type array_like<'a> = Js_array2.array_like<'a>

A type used to describe JavaScript objects that are like an array or are iterable.

from

let from: array_like<'a> => array<'a>

Creates a shallow copy of an array from an array-like object. See Array.from on MDN.

RES
let strArr = Js.String.castToArrayLike("abcd")
Js.Array.from(strArr) == ["a", "b", "c", "d"]

fromMap

let fromMap: (array_like<'a>, 'a => 'b) => array<'b>

Creates a new array by applying a function (the second argument) to each item in the array_like first argument. See Array.from on MDN.

RES
let strArr = Js.String.castToArrayLike("abcd")
let code = s => Js.String.charCodeAt(0, s)
Js.Array.fromMap(strArr, code) == [97.0, 98.0, 99.0, 100.0]

isArray

let isArray: 'a => bool

Returns true if its argument is an array; false otherwise. This is a runtime check, which is why the second example returns true---a list is internally represented as a nested JavaScript array.

RES
Js.Array.isArray([5, 2, 3, 1, 4]) == true
Js.Array.isArray(list{5, 2, 3, 1, 4}) == true
Js.Array.isArray("abcd") == false

length

let length: array<'a> => int

Returns the number of elements in the array. See Array.length on MDN.

copyWithin

let copyWithin: (~to_: int, t<'a>) => t<'a>

Copies from the first element in the given array to the designated ~to_ position, returning the resulting array. This function modifies the original array. See Array.copyWithin on MDN.

RES
let arr = [100, 101, 102, 103, 104]
Js.Array.copyWithin(~to_=2, arr) == [100, 101, 100, 101, 102]
arr == [100, 101, 100, 101, 102]

copyWithinFrom

let copyWithinFrom: (~to_: int, ~from: int, t<'a>) => t<'a>

Copies starting at element ~from in the given array to the designated ~to_ position, returning the resulting array. This function modifies the original array. See Array.copyWithin on MDN.

RES
let arr = [100, 101, 102, 103, 104]
Js.Array.copyWithinFrom(~from=2, ~to_=0, arr) == [102, 103, 104, 103, 104]
arr == [102, 103, 104, 103, 104]

copyWithinFromRange

let copyWithinFromRange: (~to_: int, ~start: int, ~end_: int, t<'a>) => t<'a>

Copies starting at element ~start in the given array up to but not including ~end_ to the designated ~to_ position, returning the resulting array. This function modifies the original array. See Array.copyWithin on MDN.

RES
let arr = [100, 101, 102, 103, 104, 105]
Js.Array.copyWithinFromRange(~start=2, ~end_=5, ~to_=1, arr) == [100, 102, 103, 104, 104, 105]
arr == [100, 102, 103, 104, 104, 105]

fillInPlace

let fillInPlace: ('a, t<'a>) => t<'a>

Sets all elements of the given array (the second arumgent) to the designated value (the first argument), returning the resulting array. This function modifies the original array. See Array.fill on MDN.

RES
let arr = [100, 101, 102, 103, 104]
Js.Array.fillInPlace(99, arr) == [99, 99, 99, 99, 99]
arr == [99, 99, 99, 99, 99]

fillFromInPlace

let fillFromInPlace: ('a, ~from: int, t<'a>) => t<'a>

Sets all elements of the given array (the last arumgent) from position ~from to the end to the designated value (the first argument), returning the resulting array. This function modifies the original array. See Array.fill on MDN.

RES
let arr = [100, 101, 102, 103, 104]
Js.Array.fillFromInPlace(99, ~from=2, arr) == [100, 101, 99, 99, 99]
arr == [100, 101, 99, 99, 99]

fillRangeInPlace

let fillRangeInPlace: ('a, ~start: int, ~end_: int, t<'a>) => t<'a>

Sets the elements of the given array (the last arumgent) from position ~start up to but not including position ~end_ to the designated value (the first argument), returning the resulting array. This function modifies the original array. See Array.fill on MDN.

RES
let arr = [100, 101, 102, 103, 104]
Js.Array.fillRangeInPlace(99, ~start=1, ~end_=4, arr) == [100, 99, 99, 99, 104]
arr == [100, 99, 99, 99, 104]

pop

let pop: t<'a> => option<'a>

If the array is not empty, removes the last element and returns it as Some(value); returns None if the array is empty. This function modifies the original array. See Array.pop on MDN.

RES
let arr = [100, 101, 102, 103, 104]
Js.Array.pop(arr) == Some(104)
arr == [100, 101, 102, 103]

let empty: array<int> = []
Js.Array.pop(empty) == None

push

let push: ('a, t<'a>) => int

Appends the given value to the array, returning the number of elements in the updated array. This function modifies the original array. See Array.push on MDN.

RES
let arr = ["ant", "bee", "cat"]
Js.Array.push("dog", arr) == 4
arr == ["ant", "bee", "cat", "dog"]

pushMany

let pushMany: (array<'a>, t<'a>) => int

Appends the values from one array (the first argument) to another (the second argument), returning the number of elements in the updated array. This function modifies the original array. See Array.push on MDN.

RES
let arr = ["ant", "bee", "cat"]
Js.Array.pushMany(["dog", "elk"], arr) == 5
arr == ["ant", "bee", "cat", "dog", "elk"]

reverseInPlace

let reverseInPlace: t<'a> => t<'a>

Returns an array with the elements of the input array in reverse order. This function modifies the original array. See Array.reverse on MDN.

RES
let arr = ["ant", "bee", "cat"]
Js.Array.reverseInPlace(arr) == ["cat", "bee", "ant"]
arr == ["cat", "bee", "ant"]

shift

let shift: t<'a> => option<'a>

If the array is not empty, removes the first element and returns it as Some(value); returns None if the array is empty. This function modifies the original array. See Array.shift on MDN.

RES
let arr = [100, 101, 102, 103, 104]
Js.Array.shift(arr) == Some(100)
arr == [101, 102, 103, 104]

let empty: array<int> = []
Js.Array.shift(empty) == None

sortInPlace

let sortInPlace: t<'a> => t<'a>

Sorts the given array in place and returns the sorted array. JavaScript sorts the array by converting the arguments to UTF-16 strings and sorting them. See the second example with sorting numbers, which does not do a numeric sort. This function modifies the original array. See Array.sort on MDN.

RES
let words = ["bee", "dog", "ant", "cat"]
Js.Array.sortInPlace(words) == ["ant", "bee", "cat", "dog"]
words == ["ant", "bee", "cat", "dog"]

let numbers = [3, 30, 10, 1, 20, 2]
Js.Array.sortInPlace(numbers) == [1, 10, 2, 20, 3, 30]
numbers == [1, 10, 2, 20, 3, 30]

sortInPlaceWith

let sortInPlaceWith: (('a, 'a) => int, t<'a>) => t<'a>

Sorts the given array in place and returns the sorted array. This function modifies the original array.

The first argument to sortInPlaceWith() is a function that compares two items from the array and returns:

  • an integer less than zero if the first item is less than the second item

  • zero if the items are equal

  • an integer greater than zero if the first item is greater than the second item

See Array.sort on MDN.

RES
// sort by word length
let words = ["horse", "aardvark", "dog", "camel"]
let byLength = (s1, s2) => Js.String.length(s1) - Js.String.length(s2)

Js.Array.sortInPlaceWith(byLength, words) == ["dog", "horse", "camel", "aardvark"]

// sort in reverse numeric order
let numbers = [3, 30, 10, 1, 20, 2]
let reverseNumeric = (n1, n2) => n2 - n1
Js.Array.sortInPlaceWith(reverseNumeric, numbers) == [30, 20, 10, 3, 2, 1]

spliceInPlace

let spliceInPlace: (~pos: int, ~remove: int, ~add: array<'a>, t<'a>) => t<'a>

Starting at position ~pos, remove ~remove elements and then add the elements from the ~add array. Returns an array consisting of the removed items. This function modifies the original array. See Array.splice on MDN.

RES
let arr = ["a", "b", "c", "d", "e", "f"]
Js.Array.spliceInPlace(~pos=2, ~remove=2, ~add=["x", "y", "z"], arr) == ["c", "d"]
arr == ["a", "b", "x", "y", "z", "e", "f"]

let arr2 = ["a", "b", "c", "d"]
Js.Array.spliceInPlace(~pos=3, ~remove=0, ~add=["x", "y"], arr2) == []
arr2 == ["a", "b", "c", "x", "y", "d"]

let arr3 = ["a", "b", "c", "d", "e", "f"]
Js.Array.spliceInPlace(~pos=9, ~remove=2, ~add=["x", "y", "z"], arr3) == []
arr3 == ["a", "b", "c", "d", "e", "f", "x", "y", "z"]

removeFromInPlace

let removeFromInPlace: (~pos: int, t<'a>) => t<'a>

Removes elements from the given array starting at position ~pos to the end of the array, returning the removed elements. This function modifies the original array. See Array.splice on MDN.

RES
let arr = ["a", "b", "c", "d", "e", "f"]
Js.Array.removeFromInPlace(~pos=4, arr) == ["e", "f"]
arr == ["a", "b", "c", "d"]

removeCountInPlace

let removeCountInPlace: (~pos: int, ~count: int, t<'a>) => t<'a>

Removes ~count elements from the given array starting at position ~pos, returning the removed elements. This function modifies the original array. See Array.splice on MDN.

RES
let arr = ["a", "b", "c", "d", "e", "f"]
Js.Array.removeCountInPlace(~pos=2, ~count=3, arr) == ["c", "d", "e"]
arr == ["a", "b", "f"]

unshift

let unshift: ('a, t<'a>) => int

Adds the given element to the array, returning the new number of elements in the array. This function modifies the original array. See Array.unshift on MDN.

RES
let arr = ["b", "c", "d"]
Js.Array.unshift("a", arr) == 4
arr == ["a", "b", "c", "d"]

unshiftMany

let unshiftMany: (array<'a>, t<'a>) => int

Adds the elements in the first array argument at the beginning of the second array argument, returning the new number of elements in the array. This function modifies the original array. See Array.unshift on MDN.

RES
let arr = ["d", "e"]
Js.Array.unshiftMany(["a", "b", "c"], arr) == 5
arr == ["a", "b", "c", "d", "e"]

append

let append: ('a, t<'a>) => t<'a>

Deprecated. append() is not type-safe. Use concat() instead.

concat

let concat: (t<'a>, t<'a>) => t<'a>

Concatenates the first array argument to the second array argument, returning a new array. The original arrays are not modified. See Array.concat on MDN.

RES
Js.Array.concat(["c", "d", "e"], ["a", "b"]) == ["a", "b", "c", "d", "e"]

concatMany

let concatMany: (array<t<'a>>, t<'a>) => t<'a>

The first argument to concatMany() is an array of arrays; these are added at the end of the second argument, returning a new array. See Array.concat on MDN.

RES
Js.Array.concatMany([["d", "e"], ["f", "g", "h"]], ["a", "b", "c"]) == [
    "a",
    "b",
    "c",
    "d",
    "e",
    "f",
    "g",
    "h",
  ]

includes

let includes: ('a, t<'a>) => bool

Returns true if the given value is in the array, false otherwise. See Array.includes on MDN.

RES
Js.Array.includes("b", ["a", "b", "c"]) == true
Js.Array.includes("x", ["a", "b", "c"]) == false

indexOf

let indexOf: ('a, t<'a>) => int

Returns the index of the first element in the array that has the given value. If the value is not in the array, returns -1. See Array.indexOf on MDN.

RES
Js.Array.indexOf(102, [100, 101, 102, 103]) == 2
Js.Array.indexOf(999, [100, 101, 102, 103]) == -1

indexOfFrom

let indexOfFrom: ('a, ~from: int, t<'a>) => int

Returns the index of the first element in the array with the given value. The search starts at position ~from. See Array.indexOf on MDN.

RES
Js.Array.indexOfFrom("a", ~from=2, ["a", "b", "a", "c", "a"]) == 2
Js.Array.indexOfFrom("a", ~from=3, ["a", "b", "a", "c", "a"]) == 4
Js.Array.indexOfFrom("b", ~from=2, ["a", "b", "a", "c", "a"]) == -1

join

let join: t<'a> => string

Deprecated. Use joinWith instead.

joinWith

let joinWith: (string, t<'a>) => string

This function converts each element of the array to a string (via JavaScript) and concatenates them, separated by the string given in the first argument, into a single string. See Array.join on MDN.

RES
Js.Array.joinWith("--", ["ant", "bee", "cat"]) == "ant--bee--cat"
Js.Array.joinWith("", ["door", "bell"]) == "doorbell"
Js.Array.joinWith("/", [2020, 9, 4]) == "2020/9/4"
Js.Array.joinWith(";", [2.5, 3.6, 3e-2]) == "2.5;3.6;0.03"

lastIndexOf

let lastIndexOf: ('a, t<'a>) => int

Returns the index of the last element in the array that has the given value. If the value is not in the array, returns -1. See Array.lastIndexOf on MDN.

RES
Js.Array.lastIndexOf("a", ["a", "b", "a", "c"]) == 2
Js.Array.lastIndexOf("x", ["a", "b", "a", "c"]) == -1

lastIndexOfFrom

let lastIndexOfFrom: ('a, ~from: int, t<'a>) => int

Returns the index of the last element in the array that has the given value, searching from position ~from down to the start of the array. If the value is not in the array, returns -1. See Array.lastIndexOf on MDN.

RES
Js.Array.lastIndexOfFrom("a", ~from=3, ["a", "b", "a", "c", "a", "d"]) == 2
Js.Array.lastIndexOfFrom("c", ~from=2, ["a", "b", "a", "c", "a", "d"]) == -1

lastIndexOf_start

let lastIndexOf_start: ('a, t<'a>) => int

Deprecated. Please use lastIndexOf.

slice

let slice: (~start: int, ~end_: int, t<'a>) => t<'a>

Returns a shallow copy of the given array from the ~start index up to but not including the ~end_ position. Negative numbers indicate an offset from the end of the array. See Array.slice on MDN.

RES
let arr = [100, 101, 102, 103, 104, 105, 106]
Js.Array.slice(~start=2, ~end_=5, arr) == [102, 103, 104]
Js.Array.slice(~start=-3, ~end_=-1, arr) == [104, 105]
Js.Array.slice(~start=9, ~end_=10, arr) == []

copy

let copy: t<'a> => t<'a>

Returns a copy of the entire array. Same as Js.Array.Slice(~start=0, ~end_=Js.Array.length(arr), arr). See Array.slice on MDN.

slice_copy

let slice_copy: (unit, t<'a>) => t<'a>

Deprecated. Please use copy.

sliceFrom

let sliceFrom: (int, t<'a>) => t<'a>

Returns a shallow copy of the given array from the given index to the end. See Array.slice on MDN.

RES
Js.Array.sliceFrom(2, [100, 101, 102, 103, 104]) == [102, 103, 104]

slice_start

let slice_start: (int, t<'a>) => t<'a>

Deprecated. Please sliceFrom.

toString

let toString: t<'a> => string

Converts the array to a string. Each element is converted to a string using JavaScript. Unlike the JavaScript Array.toString(), all elements in a ReasonML array must have the same type. See Array.toString on MDN.

RES
Js.Array.toString([3.5, 4.6, 7.8]) == "3.5,4.6,7.8"
Js.Array.toString(["a", "b", "c"]) == "a,b,c"

toLocaleString

let toLocaleString: t<'a> => string

Converts the array to a string using the conventions of the current locale. Each element is converted to a string using JavaScript. Unlike the JavaScript Array.toLocaleString(), all elements in a ReasonML array must have the same type. See Array.toLocaleString on MDN.

RES
Js.Array.toLocaleString([Js.Date.make()])
// returns "3/19/2020, 10:52:11 AM" for locale en_US.utf8
// returns "2020-3-19 10:52:11" for locale de_DE.utf8

every

let every: ('a => bool, t<'a>) => bool

The first argument to every() is a predicate function that returns a boolean. The every() function returns true if the predicate function is true for all items in the given array. If given an empty array, returns true. See Array.every on MDN.

RES
let isEven = x => mod(x, 2) == 0
Js.Array.every(isEven, [6, 22, 8, 4]) == true
Js.Array.every(isEven, [6, 22, 7, 4]) == false

everyi

let everyi: (('a, int) => bool, t<'a>) => bool

The first argument to everyi() is a predicate function with two arguments: an array element and that element’s index; it returns a boolean. The everyi() function returns true if the predicate function is true for all items in the given array. If given an empty array, returns true. See Array.every on MDN.

RES
// determine if all even-index items are positive
let evenIndexPositive = (item, index) => mod(index, 2) == 0 ? item > 0 : true

Js.Array.everyi(evenIndexPositive, [6, -3, 5, 8]) == true
Js.Array.everyi(evenIndexPositive, [6, 3, -5, 8]) == false

filter

let filter: ('a => bool, t<'a>) => t<'a>

Applies the given predicate function to each element in the array; the result is an array of those elements for which the predicate function returned true. See Array.filter on MDN.

RES
let nonEmpty = s => s != ""
Js.Array.filter(nonEmpty, ["abc", "", "", "def", "ghi"]) == ["abc", "def", "ghi"]

filteri

let filteri: (('a, int) => bool, t<'a>) => t<'a>

Each element of the given array are passed to the predicate function. The return value is an array of all those elements for which the predicate function returned true. See Array.filter on MDN.

RES
// keep only positive elements at odd indices
let positiveOddElement = (item, index) => mod(index, 2) == 1 && item > 0

Js.Array.filteri(positiveOddElement, [6, 3, 5, 8, 7, -4, 1]) == [3, 8]

find

let find: ('a => bool, t<'a>) => option<'a>

Returns Some(value) for the first element in the array that satisifies the given predicate function, or None if no element satisifies the predicate. See Array.find on MDN.

RES
// find first negative element
Js.Array.find(x => x < 0, [33, 22, -55, 77, -44]) == Some(-55)
Js.Array.find(x => x < 0, [33, 22, 55, 77, 44]) == None

findi

let findi: (('a, int) => bool, t<'a>) => option<'a>

Returns Some(value) for the first element in the array that satisifies the given predicate function, or None if no element satisifies the predicate. The predicate function takes an array element and an index as its parameters. See Array.find on MDN.

RES
// find first positive item at an odd index
let positiveOddElement = (item, index) => mod(index, 2) == 1 && item > 0

Js.Array.findi(positiveOddElement, [66, -33, 55, 88, 22]) == Some(88)
Js.Array.findi(positiveOddElement, [66, -33, 55, -88, 22]) == None

findIndex

let findIndex: ('a => bool, t<'a>) => int

Returns the index of the first element in the array that satisifies the given predicate function, or -1 if no element satisifies the predicate. See Array.find on MDN.

RES
Js.Array.findIndex(x => x < 0, [33, 22, -55, 77, -44]) == 2
Js.Array.findIndex(x => x < 0, [33, 22, 55, 77, 44]) == -1

findIndexi

let findIndexi: (('a, int) => bool, t<'a>) => int

Returns Some(value) for the first element in the array that satisifies the given predicate function, or None if no element satisifies the predicate. The predicate function takes an array element and an index as its parameters. See Array.find on MDN.

RES
// find index of first positive item at an odd index
let positiveOddElement = (item, index) => mod(index, 2) == 1 && item > 0

Js.Array.findIndexi(positiveOddElement, [66, -33, 55, 88, 22]) == 3
Js.Array.findIndexi(positiveOddElement, [66, -33, 55, -88, 22]) == -1

forEach

let forEach: ('a => unit, t<'a>) => unit

The forEach() function applies the function given as the first argument to each element in the array. The function you provide returns unit, and the forEach() function also returns unit. You use forEach() when you need to process each element in the array but not return any new array or value; for example, to print the items in an array. See Array.forEach on MDN.

RES
// display all elements in an array
Js.Array.forEach(x => Js.log(x), ["a", "b", "c"]) == ()

forEachi

let forEachi: (('a, int) => unit, t<'a>) => unit

The forEachi() function applies the function given as the first argument to each element in the array. The function you provide takes an item in the array and its index number, and returns unit. The forEachi() function also returns unit. You use forEachi() when you need to process each element in the array but not return any new array or value; for example, to print the items in an array. See Array.forEach on MDN.

RES
// display all elements in an array as a numbered list
Js.Array.forEachi((item, index) => Js.log2(index + 1, item), ["a", "b", "c"]) == ()

map

let map: ('a => 'b, t<'a>) => t<'b>

Applies the function (given as the first argument) to each item in the array, returning a new array. The result array does not have to have elements of the same type as the input array. See Array.map on MDN.

RES
Js.Array.map(x => x * x, [12, 4, 8]) == [144, 16, 64]
Js.Array.map(Js.String.length, ["animal", "vegetable", "mineral"]) == [6, 9, 7]

mapi

let mapi: (('a, int) => 'b, t<'a>) => t<'b>

Applies the function (given as the first argument) to each item in the array, returning a new array. The function acceps two arguments: an item from the array and its index number. The result array does not have to have elements of the same type as the input array. See Array.map on MDN.

RES
// multiply each item in array by its position
let product = (item, index) => item * index
Js.Array.mapi(product, [10, 11, 12]) == [0, 11, 24]

reduce

let reduce: (('b, 'a) => 'b, 'b, t<'a>) => 'b

The reduce() function takes three parameters: a reducer function, a beginning accumulator value, and an array. The reducer function has two parameters: an accumulated value and an element of the array.

reduce() first calls the reducer function with the beginning value and the first element in the array. The result becomes the new accumulator value, which is passed in to the reducer function along with the second element in the array. reduce() proceeds through the array, passing in the result of each stage as the accumulator to the reducer function.

When all array elements are processed, the final value of the accumulator becomes the return value of reduce(). See Array.reduce on MDN.

RES
let sumOfSquares = (accumulator, item) => accumulator + item * item

Js.Array.reduce(sumOfSquares, 0, [10, 2, 4]) == 120
Js.Array.reduce(\"*", 1, [10, 2, 4]) == 80
Js.Array.reduce(
  (acc, item) => acc + Js.String.length(item),
  0,
  ["animal", "vegetable", "mineral"],
) == 22 // 6 + 9 + 7
Js.Array.reduce((acc, item) => item /. acc, 1.0, [2.0, 4.0]) == 2.0 // 4.0 / (2.0 / 1.0)

reducei

let reducei: (('b, 'a, int) => 'b, 'b, t<'a>) => 'b

The reducei() function takes three parameters: a reducer function, a beginning accumulator value, and an array. The reducer function has three parameters: an accumulated value, an element of the array, and the index of that element.

reducei() first calls the reducer function with the beginning value, the first element in the array, and zero (its index). The result becomes the new accumulator value, which is passed to the reducer function along with the second element in the array and one (its index). reducei() proceeds from left to right through the array, passing in the result of each stage as the accumulator to the reducer function.

When all array elements are processed, the final value of the accumulator becomes the return value of reducei(). See Array.reduce on MDN.

RES
// find sum of even-index elements in array
let sumOfEvens = (accumulator, item, index) =>
  if mod(index, 2) == 0 {
    accumulator + item
  } else {
    accumulator
  }

Js.Array.reducei(sumOfEvens, 0, [2, 5, 1, 4, 3]) == 6

reduceRight

let reduceRight: (('b, 'a) => 'b, 'b, t<'a>) => 'b

The reduceRight() function takes three parameters: a reducer function, a beginning accumulator value, and an array. The reducer function has two parameters: an accumulated value and an element of the array.

reduceRight() first calls the reducer function with the beginning value and the last element in the array. The result becomes the new accumulator value, which is passed in to the reducer function along with the next-to-last element in the array. reduceRight() proceeds from right to left through the array, passing in the result of each stage as the accumulator to the reducer function.

When all array elements are processed, the final value of the accumulator becomes the return value of reduceRight(). See Array.reduceRight on MDN.

NOTE: In many cases, reduce() and reduceRight() give the same result. However, see the last example here and compare it to the example from reduce(), where order makes a difference.

RES
let sumOfSquares = (accumulator, item) => accumulator + item * item

Js.Array.reduceRight(sumOfSquares, 0, [10, 2, 4]) == 120
Js.Array.reduceRight((acc, item) => item /. acc, 1.0, [2.0, 4.0]) == 0.5 // 2.0 / (4.0 / 1.0)

reduceRighti

let reduceRighti: (('b, 'a, int) => 'b, 'b, t<'a>) => 'b

The reduceRighti() function takes three parameters: a reducer function, a beginning accumulator value, and an array. The reducer function has three parameters: an accumulated value, an element of the array, and the index of that element. reduceRighti() first calls the reducer function with the beginning value, the last element in the array, and its index (length of array minus one). The result becomes the new accumulator value, which is passed in to the reducer function along with the second element in the array and one (its index). reduceRighti() proceeds from right to left through the array, passing in the result of each stage as the accumulator to the reducer function.

When all array elements are processed, the final value of the accumulator becomes the return value of reduceRighti(). See Array.reduceRight on MDN.

NOTE: In many cases, reducei() and reduceRighti() give the same result. However, there are cases where the order in which items are processed makes a difference.

RES
// find sum of even-index elements in array
let sumOfEvens = (accumulator, item, index) =>
  if mod(index, 2) == 0 {
    accumulator + item
  } else {
    accumulator
  }

Js.Array.reduceRighti(sumOfEvens, 0, [2, 5, 1, 4, 3]) == 6

some

let some: ('a => bool, t<'a>) => bool

Returns true if the predicate function given as the first argument to some() returns true for any element in the array; false otherwise.

RES
let isEven = x => mod(x, 2) == 0

Js.Array.some(isEven, [3, 7, 5, 2, 9]) == true
Js.Array.some(isEven, [3, 7, 5, 1, 9]) == false

somei

let somei: (('a, int) => bool, t<'a>) => bool

Returns true if the predicate function given as the first argument to somei() returns true for any element in the array; false otherwise. The predicate function has two arguments: an item from the array and the index value

RES
// Does any string in the array
// have the same length as its index?

let sameLength = (str, index) => Js.String.length(str) == index

// "ef" has length 2 and is it at index 2
Js.Array.somei(sameLength, ["ab", "cd", "ef", "gh"]) == true
// no item has the same length as its index
Js.Array.somei(sameLength, ["a", "bc", "def", "gh"]) == false

unsafe_get

let unsafe_get: (array<'a>, int) => 'a

Returns the value at the given position in the array if the position is in bounds; returns the JavaScript value undefined otherwise.

RES
let arr = [100, 101, 102, 103]
Js.Array.unsafe_get(arr, 3) == 103
Js.Array.unsafe_get(arr, 4) // returns undefined

unsafe_set

let unsafe_set: (array<'a>, int, 'a) => unit

Sets the value at the given position in the array if the position is in bounds. If the index is out of bounds, well, “here there be dragons.“ This function modifies the original array.

RES
let arr = [100, 101, 102, 103]
Js.Array.unsafe_set(arr, 3, 99)
// result is [100, 101, 102, 99]

Js.Array.unsafe_set(arr, 4, 88)
// result is [100, 101, 102, 99, 88]

Js.Array.unsafe_set(arr, 6, 77)
// result is [100, 101, 102, 99, 88, <1 empty item>, 77]

Js.Array.unsafe_set(arr, -1, 66)
// you don't want to know.