Chrome Extension - Simple Content Script for running js on any page

Question

How can I write a simple Chrome Extension content script that will execute JavaScript (for example alert(\"hello\");) on every page load?

So when I navigate

Answer 1

If all you need is to alert hello on every page load or reload, below is a simple demo: Manifest.json:

{
    "name": "Highlight some phrases",
    "description": "Hightlight some pre defined text after page loads",
    "version": "0.1",
    "permissions": [
        "tabs","<all_urls>"
    ],
    "browser_action": {
        "default_icon": "icon.png"
    },
    "content_scripts": [
        {
        "matches": [
            "http://*/*",
            "https://*/*"
            ],
        "js": ["content.js"],
        "run_at": "document_end"    // Pay attention to this line
        }
    ], 
    "manifest_version": 2
}

and content.js:

// alert("hello");
document.body.style.background = 'yellow';

Yes, that's enough.
And of course, don't forget to add an icon named icon.png at the same directory with these two files, then test it in Google Chrome.