Why [] == [] is false in JavaScript?

Question

I am working on a part of the code where I have an array which looks like [[data]]. The data is rendered on the server side through the Django template engine. So my code looks like this:

var data = {{ series|safe }}; 
// data will be [[]] if no data is present
if (data ==[[]])
  console.log('no data');

The if always returns false. That means in [[]] == [[]] is false and my test shows that []==[] is false as well.

Any descriptions would be appreciated.

Answer 1

Because [] creates a new array, so you are comparing one array object with another array object.

It's not the contents of the arrays that is compared, the object references are compared. They are not equal because it's not the same object instance.

Answer 2

Because Arrays are reference type, meaning, if for example you make an array

let a = [1,2,3,4,5];

let b = a;

the b is actually just a reference of array a, so if you compare them

a===b is true,

because they are basically link together.. So if you change something to array b it will also going to be change to array a,

b[0] = "test";

array a now is ["test",2,3,4,5];

But if you do this this

let a = [1,2,3,4,5];

let b = a.slice(0);

and then compare them

a===b is false

because now they are both different Arrays, meaning if you change the Array b, it will not affect the Array a

b[0] ="hello";

Array a is still [1,2,3,4,5]

while array b is now ["hello",2,3,4,5]

that is also what happen when you compare the []===[] is false

Because basically what you are asking to JavaScript is if they are the same Array which is not