How do you check whether a value is an array in JavaScript?

Arrays are a special kind of object, so typeof cannot tell them apart from a plain object: both say "object". You need a check that knows about arrays specifically.

Options:

1. Array.isArray(value): the right answer. Built for exactly this job, and works everywhere.
2. value instanceof Array: works in most cases, but fails when the array came from a different window, such as an iframe, because each window has its own Array constructor.
3. Object.prototype.toString.call(value) === "[object Array]": an older trick from before ES5.

const marks = [78, 91, 66];
const student = { name: "Neha", marks: 3 };
const arrayLike = { 0: "a", 1: "b", length: 2 };

console.log(typeof marks);                 // Output: object
console.log(Array.isArray(marks));         // Output: true
console.log(Array.isArray(student));       // Output: false
console.log(Array.isArray(arrayLike));     // Output: false
console.log(Object.prototype.toString.call(marks)); // Output: [object Array]
console.log(Array.isArray(Array.from(arrayLike)));  // Output: true

An object with a length property is "array-like" but not an array. Array.from turns it into a real one.

• Checking value.length to decide it is an array; strings and array-like objects have length too.