Dmitri Pavlutin
Thoughts on Frontend development

3 Ways to Detect an Array in JavaScript

Posted June 30, 2020

Checking whether a value is an array in JavaScript is necessary when a variable is expected to be an array, but it could be a plain object or even a primitive.

In this post, you’ll find 3 good ways to detect an array instance in JavaScript.

1. Array.isArray(value)

If you don’t have time to read the entire post, here’s a good way to detect an array in JavaScript: just use Array.isArray(value) utility function.

But if you want to know more, let’s continue.

Here are a few examples of using Array.isArray():

const array = [1, 2, 3];
const object = { message: 'Hello!' };
const string = 'Hello!';
const empty = null;

Array.isArray(array);  // => trueArray.isArray(object); // => false
Array.isArray(string); // => false
Array.isArray(empty);  // => false

Array.isArray() has a good browser support. It’s the recommended way to check for an array in JavaScript.

Array.isArray(value) utility function returns true if value is an array.

2. value instanceof Array

An array is an object. And like any object in JavaScript, the array instance has a constructor function — Array.

Array invoked as a constructor (prefixed with new keyword) creates array instances:

const array = new Array(1, 2, 3);

array; // => [1, 2, 3]

Moreover, the array instance exposes its constructor using a property .constructor:

const array = [1, 2, 3];

array.constructor === Array; // => true

What is the operator that verifies whether a function is the constructor of an instance? instanceof!

Now emerges the next way to detect an array: value instanceof Array evaluates to true if value is an array.

Let’s see a few examples:

const array = [1, 2, 3];
const object = { message: 'Hello!' };
const string = 'Hello!';
const empty = null;

array  instanceof Array; // => trueobject instanceof Array; // => false
string instanceof Array; // => false
empty  instanceof Array; // => false

Resuming:

value instanceof Array expressions evaluates to true if value is an array.

Note: value instanceof Array evaluates to false when value is an array created in a different iframe than the Array constructor function. If you’re not writing cross-frames JavaScript, use this approach without worries.

3. ({}).toString.call(value)

toString() method of a plain JavaScript object returns '[object <type>]', where <type> is the object type it was called upon. Take a look at the specification which indicates how exactly Object.prototype.toString() works.

In the simplest case, when toString() method is invoked directly on a plain object, it returns '[object Object]':

({}).toString(); // => '[object Object]'

However, you can use an indirect call of toString() on different types of values, and the method returns the corresponding string representation of the type:

const array = [1, 2, 3];
const object = { message: 'Hello' };
const string = 'Hello!';
const empty = null;

({}).toString.call(array);   // => '[object Array]'({}).toString.call(object);  // => '[object Object]'
({}).toString.call(string);  // => '[object String]'
({}).toString.call(empty);   // => '[object Null]'

Now you can spot the idea: ({}).toString.call(value) equals to '[object Array]' if value is an array.

const array = [1, 2, 3];

({}).toString.call(array) === '[object Array]'; // => true

In conclusion:

({}).toString.call(value) === '[object Array]' expression evaluates to true if value is an array.

4. Summary

To detect an array my recommendation is to use Array.isArray(value). The function returns a boolean whether value is an array. Simple as is.

Another approach is value instanceof Array expression that evaluates to true if value is an array. This approach uses the idea that Array function is the constructor of the arrays.

Finally, not the most aesthetic approach is to use the expression ({}).toString.call(value) === '[object Array]' that is true if value is an array.

Object.prototype.toString() returns the type representation of the object it was invoked upon, and the type representation of arrays is '[object Array]'.

What is your favorite way to detect arrays in JavaScript?

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About Dmitri Pavlutin

I'm a passionate software developer, tech writer and coach. My daily routine consists of (but not limited to) drinking coffee, coding, writing, coaching, overcoming boredom 😉.