API / JavaScript / Js / Array-2

Array2

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

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

RE
let isEven = (x) => {x mod 2 == 0};
let square = (x) => {x * x};
let result = Js.Array2.(
  reduce(map(filter([|5, 2, 3, 4, 1|], isEven), square), (+), 0)
  );

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

RE
let isEven = (x) => {x mod 2 == 0};
let square = (x) => {x * x};
let result = Js.Array2.(
    [|5, 2, 3, 4, 1|] ->
    filter(isEven) ->
    map(square) ->
    reduce((+), 0)
  );

type t('a) = array('a);

The type used to describe a JavaScript array.

type array_like('a);

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

let from: array_like('a) => array('b);

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

RE
let strArr = Js.String.castToArrayLike("abcd");
Js.Array2.from(strArr) == [|"a", "b", "c", "d"|];

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.

RE
let strArr = Js.String.castToArrayLike("abcd");
let code = (s) => {Js.String.charCodeAt(0, s)};
Js.Array2.fromMap(strArr, code) == [|97.0, 98.0, 99.0, 100.0|];

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.

RE
Js.Array2.isArray([|5, 2, 3, 1, 4|]) == true;
Js.Array2.isArray([5, 2, 3, 1, 4]) == true;
Js.Array2.isArray("abcd") == false;

let length: array('a) => int;

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

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

RE
let arr = [|100, 101, 102, 103, 104|];
Js.Array2.copyWithin(arr, ~to_=2)
  == [|100, 101, 100, 101, 102|];
arr == [|100, 101, 100, 101, 102|];

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

RE
let arr = [|100, 101, 102, 103, 104|];
Js.Array2.copyWithinFrom(arr, ~from=2, ~to_=0)
  == [|102, 103, 104, 103, 104|];
arr == [|102, 103, 104, 103, 104|];

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

RE
let arr = [|100, 101, 102, 103, 104, 105|];
Js.Array2.copyWithinFromRange(arr, ~start=2, ~end_=5, ~to_=1)
  == [|100, 102, 103, 104, 104, 105|];
arr == [|100, 102, 103, 104, 104, 105|];

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

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

RE
let arr = [|100, 101, 102, 103, 104|];
Js.Array2.fillInPlace(arr, 99)
   == [|99, 99, 99, 99, 99|];
arr == [|99, 99, 99, 99, 99|];

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

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

RE
let arr = [|100, 101, 102, 103, 104|];
Js.Array2.fillFromInPlace(arr, 99, ~from=2)
   == [|100, 101, 99, 99, 99|];
arr == [|100, 101, 99, 99, 99|];

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

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

RE
let arr = [|100, 101, 102, 103, 104|];
Js.Array2.fillRangeInPlace(arr, 99, ~start=1, ~end_=4)
  == [|100, 99, 99, 99, 104|];
arr == [|100, 99, 99, 99, 104|];

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.

RE
let arr = [|100, 101, 102, 103, 104|];
Js.Array2.pop(arr) == Some(104);
arr == [|100, 101, 102, 103|];

let empty: array(int) = [| |];
Js.Array2.pop(empty) == None;

let push: (t('a), '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.

RE
let arr = [|"ant", "bee", "cat"|];
Js.Array2.push(arr, "dog") == 4;
arr == [|"ant", "bee", "cat", "dog"|];

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

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

RE
let arr = [|"ant", "bee", "cat"|];
Js.Array2.pushMany(arr, [|"dog", "elk"|]) == 5;
arr == [|"ant", "bee", "cat", "dog", "elk"|];

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.

RE
let arr = [|"ant", "bee", "cat"|];
Js.Array2.reverseInPlace(arr) == [|"cat", "bee", "ant"|];
arr == [|"cat", "bee", "ant"|];

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.

RE
let arr = [|100, 101, 102, 103, 104|];
Js.Array2.shift(arr) == Some(100);
arr == [|101, 102, 103, 104|];

let empty: array(int) = [| |];
Js.Array2.shift(empty) == None;

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.

RE
let words = [|"bee", "dog", "ant", "cat"|];
Js.Array2.sortInPlace(words) == [|"ant", "bee", "cat", "dog"|];
words == [|"ant", "bee", "cat", "dog"|];

let numbers = [|3, 30, 10, 1, 20, 2|];
Js.Array2.sortInPlace(numbers) == [|1, 10, 2, 20, 3, 30|];
numbers == [|1, 10, 2, 20, 3, 30|];

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

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

Js.Array2.sortInPlaceWith(words, byLength)
  == [|"dog", "horse", "camel", "aardvark"|];
  
// sort in reverse numeric order
let numbers = [|3, 30, 10, 1, 20, 2|];
let reverseNumeric = (n1, n2) => {n2 - n1};
Js.Array2.sortInPlaceWith(numbers, reverseNumeric)
  == [|30, 20, 10, 3, 2, 1|];

let spliceInPlace: (t('a), ~pos: int, ~remove: int, ~add: array('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.

RE
let arr = [|"a", "b", "c", "d", "e", "f"|];
Js.Array2.spliceInPlace(arr, ~pos=2, ~remove=2, ~add=[|"x", "y", "z"|]) == [|"c", "d"|];
arr == [|"a", "b", "x", "y", "z", "e", "f"|];

let arr2 = [|"a", "b", "c", "d"|];
Js.Array2.spliceInPlace(arr2, ~pos=3, ~remove=0, ~add=[|"x", "y"|]) == [||];
arr2 == [|"a", "b", "c", "x", "y", "d"|];

let arr3 = [|"a", "b", "c", "d", "e", "f"|];
Js.Array2.spliceInPlace(arr3, ~pos=9, ~remove=2, ~add=[|"x", "y", "z"|]) == [||];
arr3 == [|"a", "b", "c", "d", "e", "f", "x", "y", "z"|];

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

RE
let arr = [|"a", "b", "c", "d", "e", "f"|];
Js.Array2.removeFromInPlace(arr, ~pos=4) == [|"e", "f"|];
arr == [|"a", "b", "c", "d"|];

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

RE
let arr = [|"a", "b", "c", "d", "e", "f"|];
Js.Array2.removeCountInPlace(arr, ~pos=2, ~count=3) == [|"c", "d", "e"|];
arr == [|"a", "b", "f"|];

let unshift: (t('a), '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.

RE
let arr = [|"b", "c", "d"|];
Js.Array2.unshift(arr, "a") == 4;
arr == [|"a", "b", "c", "d"|];

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

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

RE
let arr = [|"d", "e"|];
Js.Array2.unshiftMany(arr, [|"a", "b", "c"|]) == 5;
arr == [|"a", "b", "c", "d", "e"|];

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

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

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

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

RE
Js.Array2.concat([|"a", "b"|], [|"c", "d", "e"|])
  == [|"a", "b", "c", "d", "e"|];

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

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

RE
Js.Array2.concatMany([|"a", "b", "c"|], [| [|"d", "e"|], [|"f", "g", "h"|] |])
  == [|"a", "b", "c", "d", "e", "f", "g", "h"|];

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

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

RE
Js.Array2.includes([|"a", "b", "c"|], "b") == true;
Js.Array2.includes([|"a", "b", "c"|], "x") == false;

let indexOf: (t('a), '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.

RE
Js.Array2.indexOf([|100, 101, 102, 103|], 102) == 2;
Js.Array2.indexOf([|100, 101, 102, 103|], 999) == -1;

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

RE
Js.Array2.indexOfFrom([|"a", "b", "a", "c", "a"|], "a", ~from=2) == 2;
Js.Array2.indexOfFrom([|"a", "b", "a", "c", "a"|], "a", ~from=3) == 4;
Js.Array2.indexOfFrom([|"a", "b", "a", "c", "a"|], "b", ~from=2) == -1;

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

RE
Js.Array2.joinWith([|"ant", "bee", "cat"|], "--") == "ant--bee--cat";
Js.Array2.joinWith([|"door", "bell"|], "") == "doorbell";
Js.Array2.joinWith([|2020, 9, 4|], "/") == "2020/9/4";
Js.Array2.joinWith([|2.5, 3.6, 3e-2|], ";") == "2.5;3.6;0.03";

let lastIndexOf: (t('a), '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.

RE
Js.Array2.lastIndexOf([|"a", "b", "a", "c"|], "a") == 2;
Js.Array2.lastIndexOf([|"a", "b", "a", "c"|], "x") == -1;

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

RE
Js.Array2.lastIndexOfFrom([|"a", "b", "a", "c", "a", "d"|], "a", ~from=3) == 2;
Js.Array2.lastIndexOfFrom([|"a", "b", "a", "c", "a", "d"|], "c", ~from=2) == -1;

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

RE
let arr = [|100, 101, 102, 103, 104, 105, 106|];
Js.Array2.slice(arr, ~start=2, ~end_=5) == [|102, 103, 104|];
Js.Array2.slice(arr, ~start=-3, ~end_=-1) == [|104, 105|];
Js.Array2.slice(arr, ~start=9, ~end_=10) == [| |];

let copy: t('a) => t('a);

Returns a copy of the entire array. Same as Js.Array2.Slice(arr, ~start=0, ~end_=Js.Array2.length(arr)). See Array.slice on MDN.

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

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

RE
Js.Array2.sliceFrom([|100, 101, 102, 103, 104|], 2) == [|102, 103, 104|];

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.

RE
Js.Array2.toString([|3.5, 4.6, 7.8|]) == "3.5,4.6,7.8";
Js.Array2.toString([|"a", "b", "c"|]) == "a,b,c";

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.

RE
Js.Array2.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

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

The first argument to every() is an array. The second argument 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.

RE
let isEven = (x) => {x mod 2 == 0};
Js.Array2.every( [|6, 22, 8, 4|], isEven) == true;
Js.Array2.every([|6, 22, 7, 4|], isEven) == false;

let everyi: (t('a), ('a, int) => bool) => bool;
The first argument to `everyi()` is an array. The second argument 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`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every) on MDN.

```re example
// determine if all even-index items are positive
let evenIndexPositive = (item, index) => 
  {(index mod 2 == 0) ? item > 0 : true;};

Js.Array2.everyi([|6, -3, 5, 8|], evenIndexPositive) == true;
Js.Array2.everyi([|6, 3, -5, 8|], evenIndexPositive) == false;

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

Applies the given predicate function (the second argument) 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.

RE
let nonEmpty = (s) => {s != ""};
Js.Array2.filter( [|"abc", "", "", "def", "ghi"|], nonEmpty)
  == [|"abc", "def", "ghi"|];

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

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

Js.Array2.filteri([|6, 3, 5, 8, 7, -4, 1|],
  positiveOddElement) == [|3, 8|];

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

RE
// find first negative element
Js.Array2.find([|33, 22, -55, 77, -44|], (x) => {x < 0}) == Some(-55);
Js.Array2.find([|33, 22, 55, 77, 44|], (x) => {x < 0}) == None;

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

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

Js.Array2.findi([|66, -33, 55, 88, 22|], positiveOddElement) == Some(88);
Js.Array2.findi([|66, -33, 55, -88, 22|], positiveOddElement) == None;

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

RE
Js.Array2.findIndex([|33, 22, -55, 77, -44|], (x) => {x < 0}) == 2;
Js.Array2.findIndex([|33, 22, 55, 77, 44|], (x) => {x < 0}) == -1;

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

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

Js.Array2.findIndexi([|66, -33, 55, 88, 22|], positiveOddElement) == 3;
Js.Array2.findIndexi([|66, -33, 55, -88, 22|], positiveOddElement) == -1;

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

The forEach() function applies the function given as the second 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.

RE
// display all elements in an array
Js.Array2.forEach([|"a", "b", "c"|], (x) => {Js.log(x)}) == ();

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

The forEachi() function applies the function given as the second 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.

RE
// display all elements in an array as a numbered list
Js.Array2.forEachi([|"a", "b", "c"|], (item, index) => {Js.log2((index + 1), item)}) == ();

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

Applies the function (the second 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.

RE
Js.Array2.map([|12, 4, 8|], (x) => {x * x}) == [|144, 16, 64|];
Js.Array2.map([|"animal", "vegetable", "mineral"|], Js.String.length)
  == [|6, 9, 7|];

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

Applies the function (the second 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.

RE
// multiply each item in array by its position
let product = (item, index) => {item * index};
Js.Array2.mapi([|10, 11, 12|], product) == [|0, 11, 24|];

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

The reduce() function takes three parameters: an array, a reducer function, and a beginning accumulator value. 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.

RE
let sumOfSquares = (accumulator, item) => {
  accumulator + (item * item);
};

Js.Array2.reduce([|10, 2, 4|], sumOfSquares, 0) == 120;
Js.Array2.reduce([|10, 2, 4|], (*), 1) == 80;
Js.Array2.reduce([|"animal", "vegetable", "mineral"|],
  (acc, item) => acc + Js.String.length(item), 0) == 22; // 6 + 9 + 7
Js.Array2.reduce([|2.0, 4.0|],
  (acc, item) => {item /. acc}, 1.0) == 2.0; // 4.0 / (2.0 / 1.0)

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

The reducei() function takes three parameters: an array, a reducer function, and a beginning accumulator value. 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.

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

Js.Array2.reducei([|2, 5, 1, 4, 3|], sumOfEvens, 0) == 6;

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

The reduceRight() function takes three parameters: an array, a reducer function, and a beginning accumulator value. 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.

RE
let sumOfSquares = (accumulator, item) => {
  accumulator + (item * item);
};

Js.Array2.reduceRight([|10, 2, 4|], sumOfSquares, 0) == 120;
Js.Array2.reduceRight([|2.0, 4.0|],
  (acc, item) => {item /. acc}, 1.0) == 0.5; // 2.0 / (4.0 / 1.0)

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

The reduceRighti() function takes three parameters: an array, a reducer function, and a beginning accumulator value. 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.

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

Js.Array2.reduceRighti([|2, 5, 1, 4, 3|], sumOfEvens, 0) == 6;

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

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

RE
let isEven = (x) => {x mod 2 == 0};

Js.Array2.some([|3, 7, 5, 2, 9|], isEven) == true;
Js.Array2.some([|3, 7, 5, 1, 9|], isEven) == false;

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

Returns true if the predicate function given as the second 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

RE
// 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.Array2.somei([|"ab", "cd", "ef", "gh"|], sameLength) == true;
// no item has the same length as its index
Js.Array2.somei([|"a", "bc", "def", "gh"|], sameLength) == false;

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.

RE
let arr = [|100, 101, 102, 103|];
Js.Array2.unsafe_get(arr, 3) == 103;
Js.Array2.unsafe_get(arr, 4); // returns undefined

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.

RE
let arr = [|100, 101, 102, 103|];
Js.Array2.unsafe_set(arr, 3, 99);
// result is [|100, 101, 102, 99|];

Js.Array2.unsafe_set(arr, 4, 88);
// result is [|100, 101, 102, 99, 88|]

Js.Array2.unsafe_set(arr, 6, 77);
// result is [|100, 101, 102, 99, 88, <1 empty item>, 77|]

Js.Array2.unsafe_set(arr, -1, 66);
// you don't want to know.