Why is using "for...in" for array iteration a bad idea?

In JavaScript, arrays are a fundamental data structure used to store multiple values in a single variable. They are commonly used for tasks such as storing lists of items, iterating over elements, and performing various operations on the data. When it comes to iterating over arrays, there are several methods available in JavaScript, such as for loops, forEach, and for...in. While for...in may seem like a convenient option, it is generally considered a bad idea to use for...in for array iteration. In this article, we will explore the reasons behind this advice and understand why alternative methods are preferred.

Understanding the for...in loop

The for...in loop is a JavaScript construct used to iterate over the properties of an object. It works by iterating over all enumerable properties of an object, including inherited properties. While arrays in JavaScript are objects themselves, they have a special internal [[Class]] property which makes them different from regular objects.

The problem with using for...in for array iteration

When using the for...in loop to iterate over an array, it does not guarantee the order in which elements will be visited. This is because the loop iterates over the enumerable properties of an object, and the order of enumerable properties in an array is not guaranteed. The for...in loop is designed for objects, where the order of properties is assumed to be irrelevant.

Additionally, when using for...in with arrays, it not only iterates over the elements of the array but also includes any enumerable properties added to the Array.prototype or Object.prototype. This means that any modifications to the prototype chain can affect the iteration, leading to unexpected results. It can also result in iterating over built-in properties and methods of arrays, which is generally not desirable.


const myArray = ['a', 'b', 'c'];
Array.prototype.customMethod = function() {
    console.log('This is a custom method');

for (const key in myArray) {
    console.log(key); // Output: 0, 1, 2, customMethod

As seen in the above example, using for...in loop on an array iterates over not only the array elements but also the added custom method from the Array.prototype.

Alternative methods for array iteration

Instead of using for...in for array iteration, there are other methods available which are specifically designed for arrays and provide more predictable behavior:

1. for loop:

A traditional for loop can be used to iterate over the array by utilizing the array's length property:

const myArray = ['a', 'b', 'c'];

for (let i = 0; i < myArray.length; i++) {

2. forEach:

The forEach method is a built-in array method introduced in ECMAScript 5. It provides a convenient way to iterate over the elements of an array:

const myArray = ['a', 'b', 'c'];

myArray.forEach(function(item) {

3. for...of loop:

The for...of loop is another option introduced in ECMAScript 6 which allows iterating over the values of an iterable object, including arrays:

const myArray = ['a', 'b', 'c'];

for (const item of myArray) {


While the for...in loop can be useful for iterating over object properties, it is not appropriate for array iteration. Using for...in on arrays can lead to unpredictable iteration order, including unexpected properties and methods, which may cause bugs and hard-to-debug issues. It is recommended to use alternative methods specifically designed for arrays, such as for loops, forEach, or for...of loop, to ensure predictable and consistent behavior when working with arrays in JavaScript.