content.js

/* behaviour is controlled via sessionStorage. Remember it stores JSON...
sessionStorage.__getUserMediaAudioError = "NotAllowedError";
sessionStorage.__getUserMediaVideoError = "NotFoundError";
sessionStorage.__filterAudioDevices = true;
sessionStorage.__filterVideoDevices = true;
sessionStorage.__filterDeviceLabels = true;
*/

// override getUserMedia to inject errors.
const origGetUserMedia = navigator.mediaDevices.getUserMedia.bind(navigator.mediaDevices);
navigator.mediaDevices.getUserMedia = async (constraints) => {
    let err;

    // for consistency with the device modifications reject with a NotFoundError.
    if (constraints.audio && sessionStorage.__filterAudioDevices) {
        err = new Error('getUserMedia error');
        err.name = 'NotFoundError';
        return Promise.reject(err);
    }

    var isScreenSharing = constraints.video && (constraints.video.mediaSource || constraints.video.mandatory && constraints.video.mandatory.chromeMediaSource);
    if (constraints.video && sessionStorage.__filterVideoDevices && !isScreenSharing) {
        err = new Error('getUserMedia error');
        err.name = 'NotFoundError';
        return Promise.reject(err);
    }

    // return errors
    if (constraints.audio && sessionStorage.__getUserMediaAudioError) {
        err = new Error('getUserMedia error');
        err.name = sessionStorage.__getUserMediaAudioError;
        return Promise.reject(err);
    }
    if (constraints.video && sessionStorage.__getUserMediaVideoError) {
        err = new Error('getUserMedia error');
        err.name = sessionStorage.__getUserMediaVideoError;
        return Promise.reject(err);
    }

    if (constraints.video && constraints.video.deviceId &&
            (constraints.video.deviceId.exact ? constraints.video.deviceId.exact.indexOf('dynamicGum:fake:') === 0 : constraints.video.deviceId.indexOf('dynamicGum:fake:') === 0)) {
        const canvas = document.createElement('canvas');
        canvas.width = 640; // TODO: actual width/height.
        canvas.height = 480;
        const ctx = canvas.getContext('2d', {alpha: false});
        ctx.fillStyle = (constraints.video.deviceId.exact ? constraints.video.deviceId.exact : constraints.video.deviceId).substr(16);
        ctx.fillRect(0,0,canvas.width, canvas.height);
        const videoStream = canvas.captureStream();
        const videoTrack = videoStream.getVideoTracks()[0];
        delete constraints.video;
        const stream = await origGetUserMedia(constraints);
        stream.addTrack(videoTrack);
        return stream;
    }
    return origGetUserMedia(constraints);
};

// override enumerateDevices to filter certain device kinds or return empty labels
// (which means no permission has been granted). Also returns empty labels
// and device ids when getUserMedia permission is denied via a session storage flag.
const origEnumerateDevices = navigator.mediaDevices.enumerateDevices.bind(navigator.mediaDevices);
navigator.mediaDevices.enumerateDevices = async () => {
    let devices = await origEnumerateDevices();
    if (sessionStorage.__filterVideoDevices) {
        devices = devices.filter((device) => device.kind !== 'videoinput');
    }
    if (sessionStorage.__filterAudioDevices) {
        devices = devices.filter((device) => device.kind !== 'audioinput');
    }

    devices = devices.map((device) => {
        const deviceWithoutLabelAndDeviceId = {
            deviceId: '',
            kind: device.kind,
            label: '',
            groupId: device.groupId,
        };

        if (device.kind === 'audioinput' && sessionStorage.__getUserMediaAudioError === 'NotAllowedError') {
            return deviceWithoutLabelAndDeviceId;
        }

        if (device.kind === 'videoinput' && sessionStorage.__getUserMediaVideoError === 'NotAllowedError') {
            return deviceWithoutLabelAndDeviceId;
        }

        if (sessionStorage.__filterDeviceLabels) {
            return deviceWithoutLabelAndDeviceId;
        }

        return device;
    });
    if (sessionStorage.__fakeVideoDevices) {
        JSON.parse(sessionStorage.__fakeVideoDevices).forEach(function(fakeDeviceSpec) {
            devices.push({
                deviceId: 'dynamicGum:fake:' + fakeDeviceSpec.color,
                kind: 'videoinput',
                label: fakeDeviceSpec.label,
                groupId: 'fake devices',
            });
        });
    }
    return devices;
};