Live Demo:

• https://www.webrtc-experiment.com/DetectRTC/

If user didn't allow webcam and/or microphone, then media-devices will be having "NULL" value for the "label" attribute. Above page will show this message: "Please invoke getUserMedia once."

PS. You can type "DetectRTC.MediaDevices" in the Chrome Console developers tool.

Note: It works only in Chrome. Firefox isn't supporting similar API yet. (Updated: Firefox supports as well)

Updated at Dec 16, 2015

Note: Following code snippet works both in Chrome and Firefox.

if (navigator.mediaDevices && navigator.mediaDevices.enumerateDevices) {    // Firefox 38+ seems having support of enumerateDevicesx    navigator.enumerateDevices = function(callback) {        navigator.mediaDevices.enumerateDevices().then(callback);    };}var MediaDevices = [];var isHTTPs = location.protocol === 'https:';var canEnumerate = false;if (typeof MediaStreamTrack !== 'undefined' && 'getSources' in MediaStreamTrack) {    canEnumerate = true;} else if (navigator.mediaDevices && !!navigator.mediaDevices.enumerateDevices) {    canEnumerate = true;}var hasMicrophone = false;var hasSpeakers = false;var hasWebcam = false;var isMicrophoneAlreadyCaptured = false;var isWebcamAlreadyCaptured = false;function checkDeviceSupport(callback) {    if (!canEnumerate) {        return;    }    if (!navigator.enumerateDevices && window.MediaStreamTrack && window.MediaStreamTrack.getSources) {        navigator.enumerateDevices = window.MediaStreamTrack.getSources.bind(window.MediaStreamTrack);    }    if (!navigator.enumerateDevices && navigator.enumerateDevices) {        navigator.enumerateDevices = navigator.enumerateDevices.bind(navigator);    }    if (!navigator.enumerateDevices) {        if (callback) {            callback();        }        return;    }    MediaDevices = [];    navigator.enumerateDevices(function(devices) {        devices.forEach(function(_device) {            var device = {};            for (var d in _device) {                device[d] = _device[d];            }            if (device.kind === 'audio') {                device.kind = 'audioinput';            }            if (device.kind === 'video') {                device.kind = 'videoinput';            }            var skip;            MediaDevices.forEach(function(d) {                if (d.id === device.id && d.kind === device.kind) {                    skip = true;                }            });            if (skip) {                return;            }            if (!device.deviceId) {                device.deviceId = device.id;            }            if (!device.id) {                device.id = device.deviceId;            }            if (!device.label) {                device.label = 'Please invoke getUserMedia once.';                if (!isHTTPs) {                    device.label = 'HTTPs is required to get label of this ' + device.kind + ' device.';                }            } else {                if (device.kind === 'videoinput' && !isWebcamAlreadyCaptured) {                    isWebcamAlreadyCaptured = true;                }                if (device.kind === 'audioinput' && !isMicrophoneAlreadyCaptured) {                    isMicrophoneAlreadyCaptured = true;                }            }            if (device.kind === 'audioinput') {                hasMicrophone = true;            }            if (device.kind === 'audiooutput') {                hasSpeakers = true;            }            if (device.kind === 'videoinput') {                hasWebcam = true;            }            // there is no 'videoouput' in the spec.            MediaDevices.push(device);        });        if (callback) {            callback();        }    });}// check for microphone/camera support!checkDeviceSupport(function() {    document.write('hasWebCam: ', hasWebcam, '<br>');    document.write('hasMicrophone: ', hasMicrophone, '<br>');    document.write('isMicrophoneAlreadyCaptured: ', isMicrophoneAlreadyCaptured, '<br>');    document.write('isWebcamAlreadyCaptured: ', isWebcamAlreadyCaptured, '<br>');});

Yes it is quite possible to detect whether a microphone and a camera is available after granting the permission,

navigator.mediaDevices.getUserMedia({ audio: true, video: true},function (stream) {     if(stream.getVideoTracks().length > 0 && stream.getAudioTracks().length > 0){         //code for when none of the devices are available                            }else{        // code for when both devices are available     }});

1)You should be using Media Recorder and understand promise

2)Check if browser support the API enumerateDevices

if (!navigator.mediaDevices || !navigator.mediaDevices.enumerateDevices) {  console.log("This browser does not support the API yet");}