JQuery and JSON array - How to check if array is empty or undefined?

Question

I need to pass php array to jquery for some tasks. The php array is created for SESSION and then json_encoded for jquery. After that I'll store that variable to window js namespace in order to use the array in my jquery script.

if(isset($_SESSION['mySession'])){
    $json_array = json_encode($php_array);
}

<script>
    <?php echo "window.json_from_php = ".$json_array; ?>
</script>

The thing i need to do is to check whether this array exist at all/is empty/undefined before doing anything. The check clause succeeds for 'undefined' operand but not for the rest. How can I check in jquery whether array is empty or not?

This is the output of the json_encoded array after initialization, when there isn't any elements in the $php_array:

string '[""]' (length=4)

And here is the jquery:

$(function() {
        var results = $('#results');
        var js_array = window.json_from_php;
        var count = js_array.length;

        if ((typeof window.json_from_php === 'undefined') || (js_array === null) || (jQuery.isEmptyObject(js_array)) || (count === 0)) {
            $('<p>Array is empty.</p>').appendTo(results);
        }
        else {
            jQuery.each(js_array,function(key,val){
                $('<ul><li id="'+key+'">'+val+'</li></ul>').appendTo(results);                  
            });
        }
    });

[""] is not an empty array. It's an array with one element: an empty string. empty array = []. — Marc B
– Marc B, Commented Nov 14, 2012 at 15:59
string '[""]' (length=4) What does this line mean? What kind of output is this? It's not a JSON structure. — Šime Vidas
– Šime Vidas, Commented Nov 14, 2012 at 16:01
I see it too. Seems like my php array isn't empty in the first place, but instead the first value is just "". Now, instead of checking if it's empty, I need to check if it's "". — art2
– art2, Commented Nov 14, 2012 at 18:36

JoeFletch · Accepted Answer · 2012-11-14 16:00:37Z

19

This is how I check for an empty/null json array.

if (js_array == undefined || js_array == null || js_array.length == 0){
    $('<p>Array is empty.</p>').appendTo(results);
}

If your json array is [""] then you may need to add something like

if (js_array == undefined || js_array == null || js_array.length == 0 || (js_array.length == 1 && js_array[0] == ""))

answered Nov 14, 2012 at 16:00

JoeFletch

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10 Comments

pimvdb Over a year ago

The first two checks are exactly the same.

Vithozor Over a year ago

I should do it like this: if (!js_array || !js_array.length){...

JoeFletch Over a year ago

@Víctor, is !js_array the equivalent of js_array == undefined || js_array == null?

Šime Vidas Over a year ago

@JoeFletch Nope. !js_array is equivalent to js_array == null || js_array === '' || js_array === 0 || js_array === -0 || js_array === false || isNaN(js_array).

Vithozor Over a year ago

@JoeFletch you can learn something interesting about truthy and falsy values here sitepoint.com/javascript-truthy-falsy , it's very useful when working with Javascript :)

|

A. Wolff · Accepted Answer · 2012-11-14 16:20:01Z

1

This should be enough:

if (typeof js_array === 'undefined' || !js_array.length) {
   $('<p>Array is empty.</p>').appendTo(results);
} else {...}

answered Nov 14, 2012 at 16:20

A. Wolff

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