I am developing a notepad application by personal development. In the process, I needed to create a chrome extension.
So in this post, I want to introduce the way to develop a simple chrome extension.

Result

In this case, the extension shows an alert message when you visit *.google.com.

The codes are Here.

What you need

You have to create only two files; manifest.json and show_alert.js.

manifest.json

Every extension needs manifest.json, which provides essential information. (ref )This time the content of it is as follows.

{
  "name": "Alert Sample",
  "description": "Chrome Extension Alert Sample",
  "version": "1.0.0",
  "manifest_version": 3,
  "content_scripts": [
    {
      "matches": ["*://*.google.com/*"],
      "js": ["show_alert.js"]
    }
  ]
}

manifest_version must be set to 2 or 3 now. (ref)

When you want to automatically run a script on some site, it is good to use content_scripts. (ref)
You can specify sites by matches consisting of scheme, host, and path. (ref)

Injected scripts into matching pages can be specified by js.

I omitted it this time, but other settings such as extension icon settings are also available in manifest.json.

show_alert.js

This contains a simple alert script like this.

window.alert("You visit 'google.com'.");

How to load the extension

After you create manifest.json and show_alert.js, you can load it in your chrome browser.

• Visit chrome://extensions/ and push Load unpacked button (if it does not show, you have to turn on Developer mode).

• Select the directory contains two files.
  

• After that, alert Sample is displayed in Extensions.

• When you visit "*://*.google.com/*" an alert message is displayed.

Summary

I introduced the way to create a simple chrome extension.
I'd like to explain a little more complicated processing in the future.
Thanks.

Reference

Japanese: Chrome拡張の作り方 (超概要)