How do I filter/modify the updated content on save using javascript?

You could start by adding:

// What to do here?
wp.data.dispatch('core/block-editor').insertDefaultBlock({content:'xxxxxx'});

This will insert a block with the specified text to the end of the blocks list when the user will hit the update button. Once, and then it will self-unsubscribe.

To run it every time the user hits the update button, you don’t want to unsubscribe(). This example will add xxxxxx every time:

const editor = wp.data.select("core/editor");
let wasSavingPost = editor.isSavingPost();
let wasAutosavingPost = editor.isAutosavingPost();

const unsubscribe = wp.data.subscribe(() => {
  const isSavingPost = editor.isSavingPost();
  const isAutosavingPost = editor.isAutosavingPost();

  const shouldTriggerAddXXXXXX =
    wasSavingPost &&
    !wasAutosavingPost &&
    !isSavingPost &&
    "page" === editor.getCurrentPost().type;

  wasSavingPost = isSavingPost;
  wasAutosavingPost = isAutosavingPost;

  if (shouldTriggerAddXXXXXX) {
    wp.data
      .dispatch("core/block-editor")
      .insertDefaultBlock({ content: "xxxxxx" });
  }
});

Note that it will not update the post in the database after last insertion of xxxxxx, because code is executed after saving.

You can learn more about this sort of thing in the Reference Guides of the Block Editor Handbook.