Question: I'm using [`content scripts`](https://developer.chrome.com/extensions/content_scripts) and I would like to inject a button to a webpage.

This is my `manifest.json`:

```
{
  "manifest_version": 2,
  "name": "my extension",
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
      {
        "matches": ["http://127.0.0.1:8000/*"],
        "js": ["script.js"],
        "run_at": "document_end"
      }
  ]
}

```

This is `popup.html`:

```
<html>
  <body>
    <input type="button" id="button" style="display:none;">
  </body>
</html>

```

And `script.js`:

```
document.body.style.backgroundColor = "yellow";
var button = document.getElementById("button");
button.style.visibility = "visible";

```

When going to `http://127.0.0.1:8000` I see the background changes to yellow, however it doesn't find button because it's not part of the webpage served by my local server. It shows this error:

```
Uncaught TypeError: Cannot read property 'style' of null

```

What should I do to inject a button on top of the content in `http://127.0.0.1:8000` (assuming I don't know its content)?

Answer: 
To insert a button, just create it in you content scripts and insert it, you needn't(shouldn't) declare it in `popup.html`

manifest.json

```
{
  "manifest_version": 2,
  "name": "my extension",
  "content_scripts": [
      {
        "matches": ["http://127.0.0.1:5000/*"],
        "js": ["script.js"]          
      }
  ]
}

```

script.js

```
document.body.style.backgroundColor = "yellow";
var button = document.createElement("button");
document.body.appendChild(button);

```
You should add a `"run_at": "document_end"` in your `content_scripts` property

In `document_end`, the script files will then be injected after the DOM is complete. This way the `document` will be able to find your lost button :)

```
"content_scripts": [
      {
        "matches": ["http://127.0.0.1:5000/*"],
        "js": ["script.js"],
        "run_at": "document_end"
      }
  ]

```