How to Write a Chrome Extension from JavaScript Snippets code
Tags: javascript, chrome, extensions
Published: 2019-02-28 17:52:46

---
title: How to Write a Chrome Extension from JavaScript Snippets code
published: true
description: Chrome Extensions can easily be built from simple JavaScript code that you might run from Browser Dev Tool snippets
tags: JavaScript, chrome, extensions
canonical: https://www.eviltester.com/blog/eviltester/2019-02-20-counterstring-extension/
---

Writing a Chrome Extension is pretty easy (getting it in the Chrome Store is much harder!). In this post I will take the snippet code I wrote to generate CounterStrings and convert it into a Chrome Extension.

I have a video showing the full process at the bottom of the post. The text is basically a summary.

You can find the code for this [Chrome Extension on Github](https://github.com/eviltester/counterstringjs)

## Counterstrings Summary

A Counterstring is a string like this `*3*5*7*9*12*15*` where the `*` represent the position in the string of the number immediately proceeding it.

- [Counterstring algorithms](https://www.eviltester.com/2018/05/counterstring-algorithms.html)
- James Bach's original writing and work on Counterstrings at [satisfice.com](http://www.satisfice.com/blog/archives/22)
- [Previous post implementing CounterStrings in JavaScript](https://www.eviltester.com/blog/eviltester/2019-02-19-counterstring-snippets/)

## A Manifest

First we need to create a `manifest.json` file.

- [developer.chrome.com/extensions/manifest](https://developer.chrome.com/extensions/manifest)

This declares the name and version of the extension, and links to the scripts being used. It is also where you declare the permissions that you need.

As you write your extensions, and you see which commands you are using, these will have the permissions required in the developer documentation.

Start with the basic manifest as documented in the [Chrome tutorial](https://developer.chrome.com/extensions/getstarted) and build it up as you add the code.

e.g. I started with

~~~~~~~~
  {
    "manifest_version": 2,
    "name": "Counterstring",
    "version": "0.1",
    "description": "simple counterstring generation"
  }
~~~~~~~~

And by the time I was finished it had become:

~~~~~~~~
{
    "manifest_version": 2,
    "name": "Counterstring",
    "version": "0.1",
    "description": "simple counterstring generation",
    "background": {
        "persistent": true,
        "scripts": ["js/background.js"]
      },
    "permissions": [
         "contextMenus",
         "activeTab"
       ],
    "icons": {
         "16": "icons/icon16x16.png",
         "48": "icons/icon48x48.png",
         "128": "icons/icon128x128.png"
         }
}
~~~~~~~~


## Background.js

The `background.js` is where you'll probably build most of your extension.

I wanted to create the simplest extension I could think of and I thought having a right click menu would be easiest. And that's really all my extension does.

I used the documentation for contenxt menus:

- [developer.chrome.com/extensions/contextMenus](https://developer.chrome.com/extensions/contextMenus)

This told me to add the `"contextMenus"` permission. Also I defined the context menu to only appear when I right click on an editable web element `"contexts" : ["editable"]`.

Because my CounterString is going to be added into that web element.

~~~~~~~~
contextMenus.createCounterString = 
    chrome.contextMenus.create(
        {"title":"Generate Counterstring",
        "contexts" : ["editable"]
        },
        function (){
            if(chrome.runtime.lastError){
                console.error(chrome.runtime.lastError.message);
            }
        }
    );
~~~~~~~~

To handle clicks on the context menu I add a Listener.

~~~~~~~~
chrome.contextMenus.onClicked.addListener(contextMenuHandler);
~~~~~~~~


And then the function that handles the context menu click uses the `chrome.tabs.executeScript` to inject some JavaScript into the active tab. 

- [About ExecuteScript](https://developer.chrome.com/extensions/tabs#method-executeScript)


~~~~~~~~
function contextMenuHandler(info, tab){

    if(info.menuItemId===contextMenus.createCounterString){
        chrome.tabs.executeScript({
            file: 'js/counterstring.js'
          });
    }
}
~~~~~~~~


## counterstring.js

The counterstring.js is the exact code that I wrote as a [snippet](https://www.eviltester.com/blog/eviltester/2019-02-19-counterstring-snippets/), but I saved it as a `counterstring.js` file

~~~~~~~~
function reverseString(reverseMe){
    return reverseMe.split("").reverse().join("");
}

function getCounterString(count){
    
    var counterString = "";

    while(count>0){

        var appendThis = "*" + reverseString(count.toString());
        
        if(appendThis.length>count){
            appendThis = appendThis.substring(0,count);
        }    

        counterString = counterString + appendThis;

        count = count - appendThis.length;
    }

    return reverseString(counterString);
}

var count = window.prompt("Counterstring Length?", "100");
var counterString = getCounterString(count);
console.log(counterString);
document.activeElement.value=counterString;
~~~~~~~~


## Persistent Background

In order to have my code injected, I needed to make the Background.js persistent. 

## Icons

Chrome expects the extensions to have icons as described here:

- [developer.chrome.com/extensions/manifest/icons](https://developer.chrome.com/extensions/manifest/icons)

You don't actually have to do this when writing your extension, it helps, but you only really need it when you want to release to the Chrome store.

## Running your extension from code

In order to run your extension you have to:

- visit chrome://extensions
- switch on Developer mode
- Click Load Unpacked
- choose the extension folder (the one with the manifest.json in it)

## Video

I've created a video showing this in action.

In the video you will see the complete process and how to debug the extension when errors occur:

- how to convert existing javascript code into a Chrome Extension
- the basic structure of an extension, manifest.json, background script and content scripts
- permissions in extensions: contextMenus, activeTab
- how to create context menus for extensions
- how to change where context menus are displayed: contexts, editable
- why to use a persistent background script
- how to dynamically inject content scripts into a page using executescript
- how to use listeners with context menus

{% youtube Olz4wo-ILwI %}


## Code

And you can find the source code on Github.

- [github.com/eviltester/counterstringjs](https://github.com/eviltester/counterstringjs)

Originally posted at [Eviltester.com](https://www.eviltester.com/blog/eviltester/2019-02-20-counterstring-extension/)