How to check if an array is ‘not empty’ in Javascript

I am working on a React project which involves making queries to a Firebase database to check if a username has been taken. The result I get from the query would be an array. When the array is not empty, it means there is a matching result, and therefore the username is not available.

I could see in the database there is a matching result and I tried to register using the same username to test, so there should be one item in the returned array.

First I set my condition as

if(array.length > 0) {

alert("Username is not available")

}

And it didn’t work, until I changed it to (with all other code unchanged)

if(!(array.length === 0)) {

alert("Username is not available")

}

Then it stared working.

I was just curious as to how the first one didn’t work as I expected. I don’t believe an array length could be less than 0. So this case, wouldn’t these 2 conditions basically do the same thing? Is there anything I am missing here?