background.js

//Only make extension icon visible on YouTube.com
const youtubeVideoWatchPageUrl = 'youtube.com/watch?v='		//lowercase matters
chrome.runtime.onInstalled.addListener(function() {
	// Replace all rules ...
	chrome.declarativeContent.onPageChanged.removeRules(undefined, () => {
		// With a new rule ...
		chrome.declarativeContent.onPageChanged.addRules([{
			conditions: [
				new chrome.declarativeContent.PageStateMatcher({
					pageUrl: { urlContains: youtubeVideoWatchPageUrl },
				})
			],
			// And shows the extension's page action
			actions: [ new chrome.declarativeContent.ShowPageAction() ]
		}])
	})
})

const urlPrerfix = 'https://www.'
const YOUTUBE_VIDEO_URL_START = urlPrerfix + youtubeVideoWatchPageUrl	//a youtube url WITHOUT the videoID
const regexToExtractVideoId = /youtube.com\/watch\?v=([^&\?]*)/i		//case insensitive regular expression matches youtube.com/watch?v=[VIDEO_ID]	& captures the video ID since the ID is either the end of the string or ends at a question mark or ampersand
const MAGIC_URL_POSTFIX = '&list=ULcxqQ59vzyTk'		//thing to append to make it play chronologically

//Run code on page action instead of popup
chrome.action.onClicked.addListener(tab=>{
	appendToUrl()
});

function appendToUrl(){
	chrome.tabs.query({'active': true, 'lastFocusedWindow': true}, tabs=>{	//Get current tab
		let currentTab = tabs[0]
		let oldURL = currentTab.url
		let regexMatch = oldURL.match(regexToExtractVideoId)
		if(!regexMatch){	//not a valid youtube video URL (prevents keyboard shortcut override)
			return
		}
		let videoId = regexMatch[1]		//get video id from regex match
		let newUrl = YOUTUBE_VIDEO_URL_START + videoId + MAGIC_URL_POSTFIX
		chrome.tabs.update(currentTab.id, {url: newUrl})	//update tab/reloads page with new location
	})
}

//Keyboard Shortcut
chrome.commands.onCommand.addListener(command => {
	if(command === 'appendToUrl'){
		appendToUrl()
	}
})

self.oninstall = () => self.skipWaiting();