Array.from()

The Array.from() static method creates a new, shallow-copied Array instance from an iterable or array-like object.

Syntax

Array.from(arrayLike)
Array.from(arrayLike, mapFn)
Array.from(arrayLike, mapFn, thisArg)

Parameters

arrayLike
- : An iterable or array-like object to convert to an array.
mapFn
- : A function to call on every element of the array. If provided, every value to be added to the array is first passed through this function, and mapFn's return value is added to the array instead. The function is called with the following arguments:
  - element
    - : The current element being processed in the array.
  - index
    - : The index of the current element being processed in the array.
thisArg
- : Value to use as this when executing mapFn.

Return value

A new Array instance.

Description

Array.from() lets you create Arrays from:

iterable objects (objects such as Map and Set); or, if the object is not iterable,
array-like objects (objects with a length property and indexed elements).

To convert an ordinary object that's not iterable or array-like to an array (by enumerating its property keys, values, or both), use Object.keys, Object.values, or Object.entries. To convert an async iterable to an array, use Array.fromAsync.

Array.from() never creates a sparse array. If the arrayLike object is missing some index properties, they become undefined in the new array.

Array.from() has an optional parameter mapFn, which allows you to execute a function on each element of the array being created, similar to map(). More clearly, Array.from(obj, mapFn, thisArg) has the same result as Array.from(obj).map(mapFn, thisArg), except that it does not create an intermediate array, and mapFn only receives two arguments (element, index) without the whole array, because the array is still under construction.

Note: This behavior is more important for typed arrays, since the intermediate array would necessarily have values truncated to fit into the appropriate type. Array.from() is implemented to have the same signature as TypedArray.from.

The Array.from() method is a generic factory method. For example, if a subclass of Array inherits the from() method, the inherited from() method will return new instances of the subclass instead of Array instances. In fact, the this value can be any constructor function that accepts a single argument representing the length of the new array. When an iterable is passed as arrayLike, the constructor is called with no arguments; when an array-like object is passed, the constructor is called with the normalized length of the array-like object. The final length will be set again when iteration finishes. If the this value is not a constructor function, the plain Array constructor is used instead.

Examples

Array from a String

Array.from("foo");
// [ "f", "o", "o" ]

Array from a Set

const set = new Set(["foo", "bar", "baz", "foo"]);
Array.from(set);
// [ "foo", "bar", "baz" ]

Array from a Map

const map = new Map([
  [1, 2],
  [2, 4],
  [4, 8],
]);
Array.from(map);
// [[1, 2], [2, 4], [4, 8]]

const mapper = new Map([
  ["1", "a"],
  ["2", "b"],
]);
Array.from(mapper.values());
// ['a', 'b'];

Array.from(mapper.keys());
// ['1', '2'];

Array from a NodeList

// Create an array based on a property of DOM Elements
const images = document.querySelectorAll("img");
const sources = Array.from(images, (image) => image.src);
const insecureSources = sources.filter((link) => link.startsWith("http://"));

Array from an Array-like object (arguments)

function f() {
  return Array.from(arguments);
}

f(1, 2, 3);

// [ 1, 2, 3 ]

Using arrow functions and Array.from()

// Using an arrow function as the map function to
// manipulate the elements
Array.from([1, 2, 3], (x) => x + x);
// [2, 4, 6]

// Generate a sequence of numbers
// Since the array is initialized with `undefined` on each position,
// the value of `v` below will be `undefined`
Array.from({ length: 5 }, (v, i) => i);
// [0, 1, 2, 3, 4]

Sequence generator (range)

// Sequence generator function (commonly referred to as "range", e.g. Clojure, PHP, etc.)
const range = (start, stop, step) =>
  Array.from({ length: (stop - start) / step + 1 }, (_, i) => start + i * step);

// Generate numbers range 0..4
range(0, 4, 1);
// [0, 1, 2, 3, 4]

// Generate numbers range 1..10 with step of 2
range(1, 10, 2);
// [1, 3, 5, 7, 9]

// Generate the alphabet using Array.from making use of it being ordered as a sequence
range("A".charCodeAt(0), "Z".charCodeAt(0), 1).map((x) =>
  String.fromCharCode(x),
);
// ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"]

Calling from() on non-array constructors

The from() method can be called on any constructor function that accepts a single argument representing the length of the new array.

function NotArray(len) {
  console.log("NotArray called with length", len);
}

// Iterable
console.log(Array.from.call(NotArray, new Set(["foo", "bar", "baz"])));
// NotArray called with length undefined
// NotArray { '0': 'foo', '1': 'bar', '2': 'baz', length: 3 }

// Array-like
console.log(Array.from.call(NotArray, { length: 1, 0: "foo" }));
// NotArray called with length 1
// NotArray { '0': 'foo', length: 1 }

When the this value is not a constructor, a plain Array object is returned.

console.log(Array.from.call({}, { length: 1, 0: "foo" })); // [ 'foo' ]