document.addEventListener('deviceready', onDeviceReady, false);
function onDeviceReady() {
   console.log('Running cordova-' + cordova.platformId + '@' + cordova.version);
   document.getElementById('deviceready').classList.add('ready');
}

document.getElementById('deviceready').addEventListener('click',deviceReadyClicked);

function deviceReadyClicked() {
   alert('deviceReady clicked');
}

<button id="testButton">TEST</button>

document.getElementById('testButton').addEventListener('click',testButtonClicked);

function testButtonClicked() {
  console.log("test button clicked");
  alert('button clicked');
}

<name>cordova-test-mytest</name>

<js-module name="cordova-test-mytest" src="www/cordova-test-mytest.js">
  <clobbers target="cordova.plugins.cordova-test-mytest" />
</js-module>

cordova.plugins.cordova-test-mytest.coolMethod();

<platform name="android">
  <config-file parent="/" target="res/xml/config.xml">
    <feature name="cordova-test-mytest">
      <param name="android-package" value="test.mytest.cordova-test-mytest" />
    </feature>
  </config-file>
  <config-file parent="/" target="AndroidManifest.xml"></config-file>
  <source-file src="src/android/cordova-test-mytest.java" target-dir="src/test/mytest/cordova-test-mytest" />
</platform>

public class cordova-test-mytest extends CordovaPlugin {
        @Override
        public boolean execute(String action, JSONArray args, CallbackContext callbackContext) throws JSONException {
            if (action.equals("coolMethod")) {
                String message = args.getString(0);
                this.coolMethod(message, callbackContext);
                return true;
            }
            return false;
        }
        private void coolMethod(String message, CallbackContext callbackContext) {
            if (message != null && message.length() > 0) {
                callbackContext.success(message);
            } else {
                callbackContext.error("Expected one non-empty string argument.");
            }
        }
}

cordova.plugins.cordova-test-mytest.coolMethod('just string example', successMtd, errorMtd);
function successMtd(message) {
    alert(message);
}
function errorMtd(message) {
    alert('Error! '+message);
}