How to change the value of a text box in Firefox Add-on SDK (Jetpack)?

I'm trying to change the contents of a text box (using the latest version of Add-on SDK, 1.05b). I'm able to get its contents but I could not find out how I can change it. Here's the relevant part of my code:

var deasciifyItem = contextMenu.Item({
  label: "Label",
  context: contextMenu.SelectorContext("input[type=text], textarea"),
  contentScript: 'self.on("click", function (node) {' +
                 'var text = node.value;' +
                  'self.postMessage(text);' +
                  '});',

  onMessage: function(text) {
    if (text.length == 0) {
      throw ("Text to convert must not be empty!");
    }

    console.log(text);
    console.log(someMyFunction(text));
    text = "A computed new value to replace the old value in text box!";
   }
  });

I can read the contents of any text box and log it to the console but how can I change its contents, e.g. the node.value by passing node.value to a function that I defined? I tried to pass node.value as a parameter to self.postMessage function but it does not work. What I'm trying to achieve is something like:

       node.value = someMyFunction(node.value);

I also tried to do that inside开发者_如何学Python

     ' node.value = someMyFunction(node.value); ' + ...

part but then it says that someMyFunction is not defined in this context (I know that it is defined because I tested that

  console.log(someMyFunction(text)); 

works).

I'm stuck at this point. Any tips? I can neither force someMyFunction into the scope of contentScript nor can I get the 'node' within 'onMessage'. What used to be very easy in previous versions of Add-on SDK turned out to be very difficult (or very unintuitive, at least) this time.

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If you can't include the entire function inside your content script (you can put your function in a separate file if it makes this easier) then you can post a message back to your content script, although this needs a function inside your content script to receive the message. See Working with Content Scripts.