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                <p id="volume"></p>


                html, body {
    width: 100%;
    height: 100%;
body {
    margin: 0;
    position: relative;
#volume {
    margin: 0;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    font-size: 100px;
    font-family: Futura, 'Century Gothic', sans-serif;


                // あらかじめgetUserMediaのポリフィルが読み込まれています

'use strict';

var ctx, analyser, frequencies, getByteFrequencyDataAverage, elVolume, draw;

window.AudioContext = window.AudioContext || window.webkitAudioContext;
ctx = new AudioContext();

analyser = ctx.createAnalyser();
frequencies = new Uint8Array(analyser.frequencyBinCount);

getByteFrequencyDataAverage = function() {
    return frequencies.reduce(function(previous, current) {
        return previous + current;
    }) / analyser.frequencyBinCount;

navigator.mediaDevices.getUserMedia({audio: true})
    .then(function(stream) {
        window.hackForMozzila = stream;
          // AnalyserNodeに接続
    .catch(function(err) {

// 音量を表示する要素
elVolume = document.getElementById('volume');
// 可能な限り高いフレームレートで音量を取得し、表示を更新する
(draw = function() {
    elVolume.innerHTML = Math.floor(getByteFrequencyDataAverage());