json getting data or calling function

Question

The function down here get's called every second, it only never outputs: console.log("noise: "+noise);

So something goes wrong and i have no idea what or how to figure it out.

function readEye() {
    console.log("readEye");

    $.getJSON('output.json', function(data){
        faceDetected = data.faceDetected;
        frameCount = data.frameCount;
        noise = data.noise;
        console.log("noise: "+noise);
    });

    // show values in devPanel
    document.getElementById('faceDetected').innerHTML = "faceDetected: "+faceDetected;

}

i think you should wait each time until $.getJSON complete loading data — mgraph
– mgraph, Commented Dec 31, 2011 at 12:23
What happens if you try to console.log(data); before faceDetected...? — Ayman Safadi
– Ayman Safadi, Commented Dec 31, 2011 at 12:24
if you load it in chrome, hit ctrl+shift+j to open the developer console and set a break point in the script tab on faceDtetcted = data.faceDtected, does it ever reach that point? — rene
– rene, Commented Dec 31, 2011 at 12:27

devnull69 · Accepted Answer · 2011-12-31 12:26:33Z

5

You stumbled upon the "asynchronous trap". $.getJSON() does NOT stop the normal program flow and will NOT wait until finished. So the following line document.getElementById('faceDetected').innerHTML = "faceDetected: "+faceDetected; will be called IMMEDIATELY with an undefined "faceDetected" variable. And this will happen every second, no matter if any of the previous requests ever finishes ....

Generally speaking you should only start a new timeout from inside the callback of an AJAX request. Additionally you should only work with those variables like "faceDetected" or "noise" inside the callback. So if you want to assign those values to a DOM element you should do it INSIDE the callback.

answered Dec 31, 2011 at 12:26

devnull69

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2 Comments

thescientist Over a year ago

in other words, everything should be handled within the callback. api.jquery.com/jQuery.getJSON review the success callback exampled in the link

clankill3r Over a year ago

But then why does it not atleast shows "noise: " in the console? Also i looked at the examples but i can't even get a callBack to work. I will make a new post since there is no space here.

BartekR · Accepted Answer · 2011-12-31 12:37:35Z

1

Also check for format of output.json - it must be well-formed and pass $.parseJSON function (http://api.jquery.com/jQuery.parseJSON/). As stated in documentation (http://api.jquery.com/jquery.getjson/):

The success callback is passed the returned data, which is typically a JavaScript object or array as defined by the JSON structure and parsed using the $.parseJSON() method. It is also passed the text status of the response.

AND

Important: As of jQuery 1.4, if the JSON file contains a syntax error, the request will usually fail silently. Avoid frequent hand-editing of JSON data for this reason.

answered Dec 31, 2011 at 12:37

BartekR

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3 Comments

clankill3r Over a year ago

my json looks like this: { "noise": 0.21046373, "framecount": 259, "faceDetected": true }

clankill3r Over a year ago

but then with returns but they never work for me in comments.

BartekR Over a year ago

your json is {"noise": 0.21046373, "framecount": 259, "faceDetected": true} but you call for data.frameCount. What if you change to data.framecount? I think this might cause silent fall.

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