Curious case of sparse array in JavaScript

I have never heard about sparse array before one of my colleagues shared this interesting issue.

const range = new Array(10).map((_, i) => i);

This code looks good to me. What can go wrong!! It should create the array of length = 10 which will have values ranging from 0 - 10.

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

Let's see....

Output

Whhaaaatttt on the earth is that output.....!!!!

The length of the array is 10 but when we try to access the value at index 0 the output is undefined. Same for other indices.

Ok. That is super weird. Next thing that usually comes to mind is this:

map isn't working. Let's use forEach.

const range = [];
new Array(10).forEach((_, i) => range.push[i]);

Output

Nooooooooooo!!

Earlier, at-least we had length 10 which was what we expected. But now we lost that too. As you can see above in the output that length of range is 0 and since length is 0 that's why no element at any indices, that's why we get undefined at range[0]

Code looks correct, then why is it that output is unexpected.

Actually all these unexpected behavior is caused by sparse array.

OK. So what on earth is sparse array ?

Enter the maze

Sparse Array

Any array will be sparse array if one of the values at any given index is empty

Empty value does not mean undefined or null. Empty mean that there is no value at all.

So in above example if I print range in chrome browser console. This will be the output:

Basically new Array(length) will create an array of specified length but all the values will be empty. It's like array has holes in it.

You can create the sparse array by following:

const range = [, , ,];
const numbers = [1, 2, , 4];
const planets = ["earth", "venus", "mars"];
delete planets[1];
// when we delete element at certain index it creates a hole

Output

Explanation of the real issue

Now we know about sparse array. So let's begin with our initial problem.

Scenario 1 : map

const range = new Array(10).map((_, i) => i);

So here new Array(10) will create a sparse array of length 10.

(10) [empty × 10]

Now what will happen when we call map on sparse array ? As per MDN Docs

Due to the algorithm defined in the specification, if the array which map was called upon is sparse, resulting array will also be sparse keeping same indices blank.

As our whole array is sparse that is why resulting array is also sparse and all the values are blank or empty.

If we have following code, result will be different

[1, 2, , 4].map((num, i) => i * num);

Output

As you can see that callback of map does not do anything on the index which is empty.

Scenario 2: forEach

const range = [];
new Array(10).forEach((_, i) => range.push[i]);

As per MDN Example

No operation for uninitialized values (sparse arrays)

Which simply means that the callback will not be invoked for empty values.

As our whole new array is sparse, callback will be skipped for all the elements and no value is pushed to range array.

Other ways to create range

With Array.from

Array.from({ length: 5 }, (_, i) => i);

With new Array(length) and fill

const a = new Array(3).fill(0).map((_, i) => i);

Output

Thank you for reading.

References

• MDN Array
• 2ality
• Freecodecamp article