Why can't I check if an array is empty with array == []?

Question

This is a question from CodeWars that is named "Count of positives/sum of negatives". It says:

If the input array is empty or null, return an empty array

To check if the array is empty, I decided to check if it was an empty array. When I try to do:

if(input == [])

I fail the test, but if I do:

if(input.length == 0)

I pass the test. An empty array should be equal to [] right? Why is there a difference, and what is the difference between these two checks?

My code is as follows:

function countPositivesSumNegatives(input) {
   var a = 0;
   var b = 0;
   if (input == null) {
      return []
   }
   if (input.length == 0) {
      return []
   }
   for (var i = 0; i < input.length; i++) {
      if (input[i] > 0) {
         a++;
      }
      if (input[i] < 0) {
         b += input[i];
      }
   }
   return [a, b]
}

put first line of method, input = input || []; and you would save lot of conditions. — A.T.
– A.T., Commented Jun 6, 2017 at 5:00
Before you consider downvoting, this question does in fact show a proper amount of research and effort. Just because you think the answer is easy doesn't necessarily mean it is. JavaScript equalities can be notoriously tricky, so cut new developers a little slack. — Patrick Roberts
– Patrick Roberts, Commented Jun 6, 2017 at 5:00
It is like comparing two empty boxes. Empty box in your left hand looks just like empty box in your right hand, but they are not the same object. However, they are both empty (length is 0). — Yeldar Kurmangaliyev
– Yeldar Kurmangaliyev, Commented Jun 6, 2017 at 5:59

Community · Accepted Answer · 2020-06-20 09:12:55Z

8

The problem is that arrays are objects. When you compare two objects, you compare their references. Per the MDN documentation:

Equality (==)

If both operands are objects, then JavaScript compares internal references which are equal when operands refer to the same object in memory.

Since two instances of arrays don't necessarily have the same reference, the check fails. Just try this in the console:

> [] == []
false

Two arrays seemingly with the same content (or lack thereof) are not equal. That's because content is not checked, but reference. These two arrays are separate instances and refer to different places in memory, thus the check evaluates to false.

On the other hand, just checking the length, and if it is zero checks if the array is empty or not. The length property signifies the amount of content in an array¹ and is part of every array. Since it is part of every array and reflects the amount of data in the array, you can use it to check if the array is empty or not.

¹ Beware, though, of sparse arrays as mentioned by RobG in the comments. Such arrays can be created with new Array(N) which will give you an empty array, but with length N.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jun 6, 2017 at 5:01

Andrew Li

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4 Comments

RobG Over a year ago

"[length] …always reflects the amount of data in the array" only when length==0, e.g. new Array(19) creates an empty array with length 19. ;-)

Andrew Li Over a year ago

@RobG Yep, I forgot about sparse arrays. I've added a note, thanks

Zze Over a year ago

@AndrewLi Great answer. Would the following be an acceptable replacement for OP? ((!input) == [])

Andrew Li Over a year ago

@Zze No. When objects are coerced to booleans, they will always be coerced to true regardless of content. It will not give the correct result for arrays that aren't empty.

YvesSaintYuran · Accepted Answer · 2020-07-18 02:52:54Z

0

null has no length

  if (input === null || input.length === 0 ) {
    return [];
    }

answered Jul 18, 2020 at 2:52

YvesSaintYuran

415 bronze badges

Comments

Muhammad Asad · Accepted Answer · 2022-07-15 09:29:19Z

0

 int[] res2 = new int[] { };

 if (input == null || input.Length == 0)
       return res2;

This code will solve your problem

answered Jul 15, 2022 at 9:29

Muhammad Asad

1

1 Comment

Community Over a year ago

Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.

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