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, Nov 14 '12 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, Nov 14 '12 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, Nov 14 '12 at 18:36

score 19 · Accepted Answer · answered Nov 14 '12 at 16:00

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 '12 at 16:00

JoeFletch

3,820
1
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2

The first two checks are exactly the same. – pimvdb Nov 14 '12 at 16:01
I should do it like this: `if (!js_array || !js_array.length){...` – Vithozor Nov 14 '12 at 16:02
@Víctor, is `!js_array` the equivalent of `js_array == undefined || js_array == null`? – JoeFletch Nov 14 '12 at 16:03
1

@JoeFletch Nope. `!js_array` is equivalent to `js_array == null || js_array === '' || js_array === 0 || js_array === -0 || js_array === false || isNaN(js_array)`. – Šime Vidas Nov 14 '12 at 16:04
@pimvdb, is `js_array == undefined` equivalent to `typeof js_array === 'undefined'` (I will have to check.) – JoeFletch Nov 14 '12 at 16:05
@JoeFletch Nope. `js_array == undefined` is equivalent to `js_array === undefined || js_array === null` (assuming that `undefined` is indeed the `undefined` value, but that's another story) – Šime Vidas Nov 14 '12 at 16:06
@Joen Fletch: Not exactly, but my point is that `== undefined` disallows `undefined`/`null` just as `== null` does. – pimvdb Nov 14 '12 at 16:06
Ok, thanks all for your feedback. I have learned something here and need to rethink some of my logic null/undefined error checking. Should I leave my answer or delete it? I feel like there are good comments here, but I have to go over some things before I edit my answer. Feedback is appreciated. – JoeFletch Nov 14 '12 at 16:12
Please leave your answer (or leave it as edited), since the last operands was a solution for me and the discussion followed is good clarification for the whole issue here. – art2 Nov 14 '12 at 18:44
4

@JoeFletch you can learn something interesting about truthy and falsy values here http://www.sitepoint.com/javascript-truthy-falsy/ , it's very useful when working with Javascript :) – Vithozor Nov 15 '12 at 07:29

score 1 · Answer 2 · answered Nov 14 '12 at 16:20

1

This should be enough:

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

answered Nov 14 '12 at 16:20

A. Wolff

74,033
9
94
155

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

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