Chrome Extension Message passing: response not sent

The accepted answer is correct, I just wanted to add sample code that simplifies this. The problem is that the API (in my view) is not well designed because it forces us developers to know if a particular message will be handled async or not. If you handle many different messages this becomes an impossible task because you never know if deep down some function a passed-in sendResponse will be called async or not. Consider this:

chrome.extension.onMessage.addListener(function (request, sender, sendResponseParam) {
if (request.method == "method1") {
    handleMethod1(sendResponse);
}

How can I know if deep down handleMethod1 the call will be async or not? How can someone that modifies handleMethod1 knows that it will break a caller by introducing something async?

My solution is this:

chrome.extension.onMessage.addListener(function (request, sender, sendResponseParam) {

    var responseStatus = { bCalled: false };

    function sendResponse(obj) {  //dummy wrapper to deal with exceptions and detect async
        try {
            sendResponseParam(obj);
        } catch (e) {
            //error handling
        }
        responseStatus.bCalled= true;
    }

    if (request.method == "method1") {
        handleMethod1(sendResponse);
    }
    else if (request.method == "method2") {
        handleMethod2(sendResponse);
    }
    ...

    if (!responseStatus.bCalled) { //if its set, the call wasn't async, else it is.
        return true;
    }

});

This automatically handles the return value, regardless of how you choose to handle the message. Note that this assumes that you never forget to call the response function. Also note that chromium could have automated this for us, I don't see why they didn't.

You can use my library https://github.com/lawlietmester/webextension to make this work in both Chrome and FF with Firefox way without callbacks.

Your code will look like:

Browser.runtime.onMessage.addListener( request => new Promise( resolve => {
    if( !request || typeof request !== 'object' || request.type !== "getUrls" ) return;

    $.ajax({
        'url': "http://localhost:3000/urls",
        'method': 'GET'
    }).then( urls => { resolve({ urls }); });
}) );

From the documentation for chrome.runtime.onMessage.addListener:

This function becomes invalid when the event listener returns, unless you return true from the event listener to indicate you wish to send a response asynchronously (this will keep the message channel open to the other end until sendResponse is called).

So you just need to add return true; after the call to getUrls to indicate that you'll call the response function asynchronously.