Question

The following code always returns “undefined”

function sendCommand(cmdJson){
    chrome.extension.sendRequest(cmdJson, function(response){
        return response;
    });
}

I’ve also tried this variant with the same result

function sendCommand(cmdJson){
    var msg;
    chrome.extension.sendRequest(cmdJson, function(response){
        msg = response;
    });
    return msg;
}

If I do an alert(response); instead of return response; I get the expected value.

Answer 1

I’m guessing that chrome.extension.sendRequest is asynchronous, in which case sendCommand doesn’t return anything. The response-handler inside sendCommand is the one that returns something, but that’s not the same, as sendCommand returning something. So when you call sendCommand it returns undefined.

Basically, sendCommand invokes the chrome.extension.sendRequest function, and then returns undefined right away, while the chrome.extension.sendRequest function is still running.

There’s no real way to make something asynchronous behave synchronously – it’s better to restructure your code.