JavaScript Array.from(), Array.of(), and Array.isArray() - Static Methods

Static Array Methods

These methods live on the Array constructor itself, not on array instances.

Array.from()

Creates a new array from any iterable or array-like object:

// From a string
Array.from("hello"); // ["h", "e", "l", "l", "o"]

// From a Set (removes duplicates)
Array.from(new Set([1, 2, 2, 3, 3])); // [1, 2, 3]

// From a Map
Array.from(new Map([["a", 1], ["b", 2]]));
// [["a", 1], ["b", 2]]

// From a NodeList
Array.from(document.querySelectorAll(".item"));

With a map function (second argument):

// Create array of specific length with computed values
Array.from({ length: 5 }, (_, i) => i + 1);
// [1, 2, 3, 4, 5]

// Generate a range
Array.from({ length: 10 }, (_, i) => i * 10);
// [0, 10, 20, 30, 40, 50, 60, 70, 80, 90]

Array.of()

Creates an array from its arguments, without the Array constructor ambiguity:

Array.of(7);       // [7] — one element: 7
Array(7);          // [empty x 7] — sparse array of length 7!

Array.of(1, 2, 3); // [1, 2, 3]
Array.of(undefined); // [undefined]

In practice, array literals [7] are preferred. Array.of() is mainly useful when the arguments are dynamic and you want to avoid the constructor pitfall.

Array.isArray()

The most reliable way to check if a value is an array:

Array.isArray([1, 2, 3]);        // true
Array.isArray("string");          // false
Array.isArray({ length: 3 });     // false
Array.isArray(new Array(3));      // true
Array.isArray(new Uint8Array(3)); // false (typed arrays are not Array)

Why not typeof or instanceof?

• typeof [] returns "object" (not useful)
• [] instanceof Array fails across iframes/realms
• Array.isArray() works correctly in all cases

Common Pattern: Normalize Input

function ensureArray(input) {
  if (Array.isArray(input)) return input;
  return [input]; // wrap single value
}
ensureArray("hello");  // ["hello"]
ensureArray([1, 2]);   // [1, 2]