---
title: JavaScript Interaction
order: 14
layout: page
---

[[advanced.javascript]]
= JavaScript Interaction

Vaadin supports two-direction JavaScript calls from and to the server-side. This
allows interfacing with JavaScript code without writing client-side integration
code.

[[advanced.javascript.calling]]
== Calling JavaScript

You can make JavaScript calls from the server-side with the
[methodname]#execute()# method in the [classname]#JavaScript# class. You can get
a [classname]#JavaScript# instance from the current [classname]#Page# object
with [methodname]#getJavaScript()#. 
//TODO Check that the API is so.


[source, java]
----
// Execute JavaScript in the currently processed page
Page.getCurrent().getJavaScript().execute("alert('Hello')");
----

The [classname]#JavaScript# class itself has a static shorthand method
[methodname]#getCurrent()# to get the instance for the currently processed page.


[source, java]
----
// Shorthand
JavaScript.getCurrent().execute("alert('Hello')");
----

The JavaScript is executed after the server request that is currently processed
returns. If multiple JavaScript calls are made during the processing of the
request, they are all executed sequentially after the request is done. Hence,
the JavaScript execution does not pause the execution of the server-side
application and you can not return values from the JavaScript.


[[advanced.javascript.callback]]
== Handling JavaScript Function Callbacks

You can make calls with JavaScript from the client-side to the server-side. This
requires that you register JavaScript call-back methods from the server-side.
You need to implement and register a [classname]#JavaScriptFunction# with
[methodname]#addFunction()# in the current [classname]#JavaScript# object. A
function requires a name, with an optional package path, which are given to the
[methodname]#addFunction()#. You only need to implement the [methodname]#call()#
method to handle calls from the client-side JavaScript.


[source, java]
----

JavaScript.getCurrent().addFunction("com.example.foo.myfunc",
                                    new JavaScriptFunction() {
    @Override
    public void call(JsonArray arguments) {
        Notification.show("Received call");
    }
});

Link link = new Link("Send Message", new ExternalResource(
        "javascript:com.example.foo.myfunc()"));
----

Parameters passed to the JavaScript method on the client-side are provided in a
[classname]#JSONArray# passed to the [methodname]#call()# method. The parameter
values can be acquired with the [methodname]#get()# method by the index of the
parameter, or any of the type-casting getters. The getter must match the type of
the passed parameter, or an exception is thrown.


[source, java]
----
JavaScript.getCurrent().addFunction("com.example.foo.myfunc",
                                    new JavaScriptFunction() {
    @Override
    public void call(JsonArray arguments) {
        try {
            String message = arguments.getString(0);
            int    value   = arguments.getInt(1);
            Notification.show("Message: " + message +
                              ", value: " + value);
        } catch (Exception e) {
            Notification.show("Error: " + e.getMessage());
        }
    }
});
        
Link link = new Link("Send Message", new ExternalResource(
  "javascript:com.example.foo.myfunc(prompt('Message'), 42)"));
----

The function callback mechanism is the same as the RPC mechanism used with
JavaScript component integration, as described in
<<dummy/../../../framework/gwt/gwt-javascript#gwt.javascript.rpc,"RPC from
JavaScript to Server-Side">>.