Chrome extension how to get value from block div?

Question

SQLite

Good time and day.
Question from a newbie:
I'm writing an extension for Chrome. It is necessary that when the extension button is clicked, it receives data from the html page and displays it in alert
Sample page code

<div class="smallfont post-title">
<strong>Получить значение дива по class</strong>
</div>

This is the manifesto

{
    "manifest_version": 2,
    "name": "superpuper",
    "description": "yop yop",
    "version": "1.0",
    "browser_action": {
        "default_icon": "calc.png",
        "default_title": "yopyop"
    },

  "icons": { "16": "calc.png",
               "48": "calc.png",
               "128": "calc.png" },
  "permissions": [
    "tabs",
    "activeTab" 
   ],

  "background": {
    "scripts": ["background.js"]
  }
}

background.js

function onExtensionClick() {
var a = document.getElementsByClassName('post-title')[0].innerHTML;
alert("a");

Button does not work when pressed

Answer 1

give me your choice