Question 395145 in Javascript Question posted in
A very good W3school tutorial can be found here.

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?

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5

Answers


  1. 0

    If there is no element in array, then array.length === 0 condition works fine.

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  2. 0

    An elegant way I feel is to use not operator

    if (!array.length) do something;

    You can find more ways here enter link description here

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  3. 0

    For me comparing a number to 0 is useless since 0 is falsy (it actually detects a novice JS programmer for me), just use:

    if(array.length) {
  alert("Username is not available")
}
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  4. 0

    Just check a.length itself

    > a=[]
[]
> if(a.length) { console.log("true") }

> a=[1]
[ 1 ]
> if(a.length) { console.log("true") }
true

> a=["1","2"]
[ '1', '2' ]
> if(a.length) { console.log("true") }
true
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  5. 0

    Your first method (array.length > 0) works, and I think it’s more readable than relying on implicit casting.

    Review the response of the call to make sure it’s returning an array or a map.

    If it doesn’t and returns a non-iterable object, !(array.length === 0) will result in true because array.length would be undefined, and indeed it’s not 0.

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