Slice of Dev Logo

Why Array(n) is Not Iterable in JavaScript

Cover Image for Why Array(n) is Not Iterable in JavaScript
Author's Profile Pic
Rajkumar Gaur


If you have come across the use case of iterating n number of times in JavaScript, chances are, you might have tried to implement the solution below (especially in react, when you need to render a JSX element n number of times).

Array(5).map(() => console.log("hello"))

If you think the above code might print “hello” 5 times, that's not the case. It is not going to print anything! Let’s look at why is that.

Sparse Arrays

When JavaScript arrays are instantiated without any elements but just with the length, elements at all the indices are filled with empty items.

These elements are not iterable with some array iteration methods like forEach , map , filter , etc. These elements will skip the empty elements and just iterate over other non-empty elements if any.

For example,

const arr = Array(5);

console.log(arr); // [ <5 empty items> ]

arr[1] = 1;
arr[3] = 3;
arr[5] = 5;

console.log(arr); // [ <1 empty item>, 1, <1 empty item>, 3, <1 empty item>, 5 ]

arr.forEach((ele) => console.log(ele)); // 1, 3, 5

If the length of an array is increased by setting the length property directly or by assigning to an index that is greater than the array’s current length, the new elements would be assigned by empty items.

const arr = [1, 2, 3];

arr.length = 5;

console.log(arr); // [ 1, 2, 3, <2 empty items> ]

arr[7] = 8;

console.log(arr); // [ 1, 2, 3, <4 empty items>, 8 ]

arr.forEach((ele) => console.log(ele)); // 1, 2, 3, 8

delete arr[1];

console.log(arr); // [ 1, <1 empty item>, 3, <4 empty items>, 8 ]

const newArr = [1, , 2, , 3];

console.log(newArr); // [ 1, <1 empty item>, 2, <1 empty item>, 3 ]

Empty items to undefined

Empty items are treated as undefined in some operations like for...of loop, accessing via index, and the spread ... operator.

const arr = [1, , 2, , 3];

for (const ele of arr) { // 1, undefined, 2, undefined, 3, undefined

console.log(...arr); // 1, undefined, 2, undefined, 3, undefined

console.log(arr[0]); // 1
console.log(arr[1]); // undefined

Let’s come back to our original problem. The alternatives that can be used for looping n number of times in a single statement.

[...Array(3)].map((e) => console.log(e)); // undefined, undefined, undefined

Array.from({ length: 3 }).map((e) => console.log(e)); // undefined, undefined, undefined


The above methods come in handy when you need to iterate for a certain number of times in a single statement, like rendering a JSX element multiple times in React.

That’s all folks! Thanks for reading.

Cover Image for React Interview Experience

React Interview Experience

This blog is about my recent React interview experiences and some interesting questions that were asked. These questions might help you prepare for your next interview. Guess The Output | useState vs useReducer | useCallback and useMemo | Redux Vs Context API | Manage The Focus Of Elements Using React | Why is useRef used | Coding Problem.

Author's Profile Pic
Rajkumar Gaur
Cover Image for Streams in NodeJS

Streams in NodeJS

Node.js Streams are an essential feature of the platform that provide an efficient way to handle data flows. They allow for processing of large volumes of data in a memory-efficient and scalable way, and can be used for a variety of purposes such as reading from and writing to files, transforming data, and combining multiple streams into a single pipeline

Author's Profile Pic
Rajkumar Gaur