cs.js

(function() {
	if (window.hasRun) {
		return;
	}
	window.hasRun = true;

	var dB = 0;
	var mono = false;

	var videoElements = document.querySelectorAll("video");
	var audioElements = document.querySelectorAll("audio");
	if (videoElements.length == 0 && audioElements.length == 0) {
		return;
	}
	var audioCtx = new AudioContext();
	var gainNode = audioCtx.createGain();
	gainNode.channelInterpretation = 'speakers';

	function connectOutput(element) {
		audioCtx.createMediaElementSource(element).connect(gainNode);
		gainNode.connect(audioCtx.destination);
	}
	videoElements.forEach(connectOutput);
	audioElements.forEach(connectOutput);

	function setVolume(dB) {
		gainNode.gain.value = Math.pow(10, dB/20);
	}

	function enableMono() {
		mono = true;
		gainNode.channelCountMode = 'explicit';
		gainNode.channelCount = 1;
	}

	function disableMono() {
		mono = false;
		gainNode.channelCountMode = 'max';
		gainNode.channelCount = 2;
	}

	browser.runtime.onMessage.addListener((message) => {
		switch (message.command) {
		case "setVolume":
			dB = message.dB;
			setVolume(dB);
			break;
		case "getVolume":
			return Promise.resolve({response: dB});
			break;
		case "setMono":
			if (message.mono) {
				enableMono();
			} else {
				disableMono();
			}
			break;
		case "getMono":
			return Promise.resolve({response: mono});
			break;
		}
	});
})();