javascript function not returning correct value

Question

I have this function that checks a UK postcode. The problem is that it never returns true or false - only undefined.

function PostcodeAnywhere_Interactive_FindByPostcode_v1_00(Key, Postcode, UserName) {

    var retval;

    $.getJSON("https://services.postcodeanywhere.co.uk/PostcodeAnywhere/Interactive/FindByPostcode/v1.00/json3.ws?",
    {
        Key: Key,
        Postcode: Postcode,
        UserName: UserName
    },
    function (response) {
        // Test for an error
        if (response.Items.length == 1 && typeof(response.Items[0].Error) != "undefined") {
            // Show the error message
            retval = false;
        } else {
            // Check if there were any items found
            if (response.Items.length == 0){
                retval = false;
            } else {
                retval = true;
            }
        }
    });

return retval;

}

To me it looks like it should always return true or false, so I can't understand where I'm going wrong. Can someone please help? Is it that the getJSON function needs time to execute?

possible duplicate of Return Value from inside of $.ajax() function -- please use the search before you ask a question. — Felix Kling
– Felix Kling, Commented Feb 13, 2012 at 16:25
Anybody else read Nick Craver's blog post about SO storage requirements? I wonder how much of the disk array is taken up by questions like this? :-) — Pointy
– Pointy, Commented Feb 13, 2012 at 16:28
simply because you return retval before setting it (asynchronous) — soju
– soju, Commented Feb 13, 2012 at 16:29
Sorry everyone for posting a question already asked - I didn't realise that $.getJSON worked in a similar way as $.ajax and that it was therefore asynchronous — SammyBlackBaron
– SammyBlackBaron, Commented Feb 13, 2012 at 16:46
Possible duplicate of Return Value from inside of $.ajax() function — user5664615
– user5664615, Commented Dec 13, 2017 at 16:19

iCollect.it Ltd · Accepted Answer · 2013-10-16 13:31:04Z

9

It returns undefined because $.getJSON runs asynchronously and for this reason retval is returned before the success function of $.getJSON is executed. If you need to use retval you must call the function that uses it in the callback

   $.getJSON("https://services.postcodeanywhere.co.uk/PostcodeAnywhere/Interactive/FindByPostcode/v1.00/json3.ws?",
    {
        Key: Key,
        Postcode: Postcode,
        UserName: UserName
    },
    function (response) {
        // Test for an error
        if (response.Items.length == 1 && typeof(response.Items[0].Error) != "undefined") {
            // Show the error message
            retval = false;
        } else {
            // Check if there were any items found
            if (response.Items.length == 0){
                retval = false;
            } else {
                retval = true;
            }
            //use retval
            do_something_with_retval(retval);
        }
    });

edited Oct 16, 2013 at 13:31

iCollect.it Ltd

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answered Feb 13, 2012 at 16:26

Nicola Peluchetti

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