Two empty arrays are not the same as one another, for they are not the same object.

var a = [];
if (a === []){
  // This will never execute
}

Use if (siteArray.length==0) to see if an array is empty, or more simply if (!siteArray.length)

For those playing at home, a non jQuery solution:

var test2 = [{a: 1},{},{}] ;  //This is empty

function isEmpty(val) {
    var len = val.length,
        i;

    if (len > 0) {
        for (i = 0; i < len; ++i) {
            if (!emptyObject(val[i])) {
                return false;
            }
        }
    }
    return true;
}

function emptyObject(o) {
    for (var key in o) {
        if (o.hasOwnProperty(key)) {
            return false;
        }
    }
    return true;
}

console.log(isEmpty(test2));

You can also do jQuery.isEmptyObject(data.RoleOwners)

check out http://api.jquery.com/jQuery.isEmptyObject/

To check if an array is either empty or not

A modern way, ES5+:

if (Array.isArray(array) && array.length) {
    // array exists and is not empty
}

An old-school way:

typeof array != "undefined"
    && array != null
    && array.length != null
    && array.length > 0

A compact way:

if (typeof array != "undefined" && array != null && array.length != null && array.length > 0) {
    // array exists and is not empty
}

A CoffeeScript way:

if array?.length > 0

Why?

Case Undefined
Undefined variable is a variable that you haven't assigned anything to it yet.

let array = new Array();     // "array" !== "array"
typeof array == "undefined"; // => true

Case Null
Generally speaking, null is state of lacking a value. For example a variable is null when you missed or failed to retrieve some data.

array = searchData();  // can't find anything
array == null;         // => true

Case Not an Array
Javascript has a dynamic type system. This means we can't guarantee what type of object a variable holds. There is a chance that we're not talking to an instance of Array.

supposedToBeArray =  new SomeObject();
typeof supposedToBeArray.length;       // => "undefined"

array = new Array();
typeof array.length;                   // => "number"

Case Empty Array
Now since we tested all other possibilities, we're talking to an instance of Array. In order to make sure it's not empty, we ask about number of elements it's holding, and making sure it has more than zero elements.

firstArray = [];
firstArray.length > 0;  // => false

secondArray = [1,2,3];
secondArray.length > 0; // => true

var jsonData = JSON.parse(responseBody);
tests['empty_or_not'] = jsonData.length === 0;

It depends on what you mean by "empty."

If you mean an object with no properties

If you mean you're getting:

 [{}]

...then madox2's answer is a good way to check. Alternatively, you might have a function to do it that you can reuse:

function isEmptyObject(obj) {
    for (const key in obj) {
        if (Object.hasOwn(obj, key)) { // Remove this if you want to check for
                                       // enumerable inherited properties
            return false;
        }
    }
    return true;
}

(Object.hasOwn was added in ES2022, but is easily polyfilled.)

If you mean [null] or similar

If you want to check specifically for undefined (note the === rather than ==) (but you won't get undefined from JSON):

if (myarray[0] === undefined)

Or specifically for null (note the === rather than ==):

if (myarray[0] === null)

Or for either of those (note the == rather than ===):

if (myarray[0] == null)

Or for any falsy value (0, "", NaN, null, undefined, or of course, false):

if (!myarray[0])

As others have suggested, it appears you don't have a need what whatsoever for

obj = JSON.parse(obj);

Finally to check emptiness of an array is easy, in your case, do this:

r.innerHTML = !obj.data.length;

or

r.innerHTML = obj.data.length === 0;

But if you really have a need to define a function for this purpose, then the following should be enough:

function isEmpty(a) {
    return a && !a.length;
}

Just test if the array is empty.

$.getJSON(url,function(json){
    if ( json.length == 0 ) {
        console.log("NO DATA!")
    }
});

Use Array.length length gives you length of the array.

var arr = [];
if(arr.length == 0){
   alert("Array is empty);
}