How do we check if an object is an array in Javascript?

In JavaScript, typeof returns "object" for arrays, which makes it unreliable for array detection. There are three better approaches: Array.isArray(), the constructor property, and instanceof.

The Problem with typeof

typeof returns "object" for arrays, objects, and null − it cannot distinguish arrays from other objects ?

let str = "Abdul Rawoof";
let arr = [1, 3, 5, 8];
let num = 90;
let dict = {a: 1, b: 2};

console.log(typeof str);   // "string"
console.log(typeof arr);   // "object"  ? not "array"
console.log(typeof num);   // "number"
console.log(typeof dict);  // "object"  ? same as array

string
object
number
object

Both the array and the object return "object", so typeof cannot tell them apart.

Using Array.isArray() (Recommended)

Array.isArray() returns true if the input is an array, false otherwise. This is the most reliable method ?

let dict = {name: "John", age: 18};
let arr = ["red", "green", "blue"];
let arr1 = [1, 2, 3, 4, 5];
let str = "hello";

console.log(Array.isArray(dict));      // false
console.log(Array.isArray(arr));       // true
console.log(Array.isArray(arr1));      // true
console.log(Array.isArray(str));       // false
console.log(Array.isArray(null));      // false
console.log(Array.isArray(undefined)); // false

false
true
true
false
false
false

Using the Constructor Property

Check if the constructor property equals Array. This works for most cases but fails on null and undefined ?

console.log("".constructor === Array);           // false
console.log({}.constructor === Array);           // false
console.log([].constructor === Array);           // true
console.log([1, "hello"].constructor === Array); // true
console.log(new Array().constructor === Array);  // true

// These would throw TypeError:
// null.constructor === Array
// undefined.constructor === Array

false
false
true
true
true

Using instanceof

The instanceof operator checks if an object is an instance of the Array constructor ?

console.log([1, 2, 3] instanceof Array);   // true
console.log({a: 1} instanceof Array);      // false
console.log("hello" instanceof Array);     // false

true
false
false

Comparison

Conclusion

Use Array.isArray() as the standard way to check if a value is an array in JavaScript. It handles all edge cases including null, undefined, and cross-frame objects. Avoid relying on typeof since it returns "object" for both arrays and plain objects.