How can one extract selected text with a context menu

Question

This is the first mechanism my chrome extension needs to perform, and I am unable to find a way to do this. I've tried using the window.getSelection().toString() / document.getSelection().toString() method, but I can only get it to work on a webpage, and I've also tried implementing an onclick function directly into the context menu object, but it did not even get the option to pop up in the context menu while I was running it. This is my manifest file so far:

{
  "manifest_version": 2,

  "name": "Menu",
  "version": "1.0",
  "icons": {
    "128": "icon128.png",
    "48": "icon48.png",
    "16": "icon16.png"
  },

  "browser_action": {
    "default_icon": "icon16.png"
  },

  "background": {
    "scripts": ["eventPage.js"],
    "persistent": false
  },

  "permissions": [
    "contextMenus",
    "tabs"
  ]

}

and this is my event page javascript file so far:

chrome.contextMenus.create({ id: "1", title: "Feature One", contexts: ["selection"] }); 

chrome.contextMenus.create({ id: "2", title: "Feature Two", contexts: ["selection"] }); 

chrome.contextMenus.create({ id: "3", title: "Feature Three", contexts: ["selection"] }); 

chrome.contextMenus.onClicked.addListener(function(clickData){

  if (clickData.menuItemId == "1" && clickData.selectionText){ 
    //alert("");
  }
  if (clickData.menuItemId == "2" && clickData.selectionText){ 
    //alert(""); 
  }
  if (clickData.menuItemId == "3" && clickData.selectionText){ 
    //alert(""); 
  } 

});

Thank you for reading my question