Saving desktopCapturer to video file in Electron

Take a look at the MediaRecorder API

You should convert that stream to binary chunks.

I answered my own question with the help of Demian's link to MediaRecorder as well as other related questions.

Below is an excerpt from magnemite with some minor simplifications and converted from TypeScript to JavaScript ES5 for better understanding to most readers.

var fs = require('fs');
var electron = require('electron');

var SECRET_KEY = 'Magnemite';

var recorder;
var blobs = [];

function startRecording() {
    var title = document.title;
    document.title = SECRET_KEY;

    electron.desktopCapturer.getSources({ types: ['window', 'screen'] }, function(error, sources) {
        if (error) throw error;
        for (let i = 0; i < sources.length; i++) {
            let src = sources[i];
            if (src.name === SECRET_KEY) {
                document.title = title;

                navigator.webkitGetUserMedia({
                    audio: false,
                    video: {
                        mandatory: {
                            chromeMediaSource: 'desktop',
                            chromeMediaSourceId: src.id,
                            minWidth: 800,
                            maxWidth: 1280,
                            minHeight: 600,
                            maxHeight: 720
                        }
                    }
                }, handleStream, handleUserMediaError);
                return;
            }
        }
    });
}

function handleStream(stream) {
    recorder = new MediaRecorder(stream);
    blobs = [];
    recorder.ondataavailable = function(event) {
        blobs.push(event.data);
    };
    recorder.start();
}

function stopRecording() {
    recorder.stop();
    toArrayBuffer(new Blob(blobs, {type: 'video/webm'}), function(ab) {
        var buffer = toBuffer(ab);
        var file = `./videos/example.webm`;
        fs.writeFile(file, buffer, function(err) {
            if (err) {
                console.error('Failed to save video ' + err);
            } else {
                console.log('Saved video: ' + file);
            }
        });
    });
}

function handleUserMediaError(e) {
    console.error('handleUserMediaError', e);
}

function toArrayBuffer(blob, cb) {
    let fileReader = new FileReader();
    fileReader.onload = function() {
        let arrayBuffer = this.result;
        cb(arrayBuffer);
    };
    fileReader.readAsArrayBuffer(blob);
}

function toBuffer(ab) {
    return Buffer.from(ab);
}

// Record for 7 seconds and save to disk
startRecording();
setTimeout(function() { stopRecording() }, 7000);

This will record the current electron window for 7 seconds and save to disk.

The desktopCapturer example shows how to get a Blob. You then need to convert the Blob to something accepted by fs.writeFile. The following is short and efficient:

fs.writeFile(filepath, Buffer.from(await blob.arrayBuffer()), ...);