script.js

const currentUrl = window.location.href
const urlParams = new URLSearchParams(window.location.search)
const options = { // Setup all possible actions and specify require arguments
	followPlayerIntoGame: ['followUserId'],
	editGameInStudio: ['editPlaceId'],
	joinGameInstance: [{param: 'placeId', regex: /\/games\/(\d+)/}, 'gameId'],
	joinPrivateGame: [{param: 'privatePlaceId', regex: /\/games\/(\d+)/}, ['privateId'], ['code']],
	joinMultiplayerGame: ['placeId']
}

const minimumParams = {
	joinPrivateGame: 2
}

if (!urlParams.get('page')) { // Don't run if Roblox analytics headers are present (ref #5)
	for (const action in options) { // Iterate actions
		const data = options[action]
		const input = []
		for (const key of data) { // Fetches required parameters into table
			const optional = Array.isArray(key)
			let paramKey = optional ? key[0] : key
			if (!optional && typeof key === 'object') {
				if (key.regex) {
					const result = key.regex.exec(currentUrl)
					if (result) {
						input.push(result[1])
						continue
					}
				}
				paramKey = key.param
			}
			if (!paramKey) continue

			const value = urlParams.get(paramKey)
			if (value != null) { input.push(value)
			} else if (optional) { input.push(null)
			}
		}
		if (input.length !== data.length) continue // Not enough values, check next action

		if (minimumParams[action]) {
			let nonNullParams = 0
			for (const param of input) if (param != null) nonNullParams += 1
			if (nonNullParams < minimumParams[action]) continue // Not enough values, check next action
		}

		let attempt = 0
		const interval = setInterval(() => {
			attempt += 1
			try {
				Roblox.GameLauncher[action](...input) // Execute
				clearInterval(interval)
				if (!urlParams.get('dontClose')) document.body.classList.add('RBLX_URL_LAUNCHER_OPENED')
			} catch(e) {
				console.log("Couldn't run the game - ", e)
			}
			if (attempt > 3) clearInterval(interval) // Stop if exceeded the limit of attempts
		}, 1000)
		break
	}
}