webextensions-examples/export-helpers/content_scripts/export.js

/*
Define a function in the content script's scope, then export it
into the page script's scope.
*/
function notify(message) {
  chrome.runtime.sendMessage({content: "Function call: " + message});
}

window.wrappedJSObject.notify = exportFunction(notify, window);

/*
Create an object that contains functions in the content script's scope,
then clone it into the page script's scope.

Because the object contains functions, the cloneInto call must include
the `cloneFunctions` option.
*/
var messenger = {
  notify: function(message) {
    chrome.runtime.sendMessage({content: "Object method call 1: " + message});
  }
};

window.wrappedJSObject.messenger = cloneInto(messenger, window, {cloneFunctions: true});

/*
Create an object in the page script's scope.
*/
function notify2(message) {
  chrome.runtime.sendMessage({content: "Object method call 2: " + message});
}

var messenger2 = createObjectIn(window.wrappedJSObject, {defineAs: "messenger2"});
messenger2.notify = exportFunction(notify2, messenger2);