navigator.mediaDevices.getUserMedia + MediaRecorder create WEBM files without duration metadata.

This library appends missing metadata section right to the file blob.

The library contains only one script fix-webm-duration.js and has no dependencies.

Syntax:

ysFixWebmDuration(blob, duration, callback = undefined, options = {});

where

• blob is Blob object with file contents from MediaRecorder
• duration is video duration in milliseconds (you should calculate it while recording the video)
• callback is callback function that will receive fixed blob. If omitted, a Promise object will be returned instead.
• options is an object of options:
  • options.logger - a callback for logging debug messages or false. The callback should accept one argument - the message string. By default, console.log will be used for logging. Passing false will disable the logging.

ysFixWebmDuration will parse and fix your file asynchronously and will call your callback once the result is ready.

If the original blob already contains duration metadata section and the duration value is not empty, the callback will receive it without any changes made.

Example:

var mediaRecorder;
var mediaParts;
var startTime;

function startRecording(stream, options) {
    mediaParts = [];
    mediaRecorder = new MediaRecorder(stream, options);
    mediaRecorder.onstop = function() {
        var duration = Date.now() - startTime;
        var buggyBlob = new Blob(mediaParts, { type: 'video/webm' });

        // v1: callback-style
        ysFixWebmDuration(buggyBlob, duration, function(fixedBlob) {
            displayResult(fixedBlob);
        });

        // v2: promise-style, disable logging
        ysFixWebmDuration(buggyBlob, duration, {logger: false})
            .then(function(fixedBlob) {
                displayResult(fixedBlob);
            });
    };
    mediaRecorder.ondataavailable = function(event) {
        var data = event.data;
        if (data && data.size > 0) {
            mediaParts.push(data);
        }
    };
    mediaRecorder.start();
    startTime = Date.now();
}

function stopRecording() {
    mediaRecorder.stop();
}

function displayResult(blob) {
    // ...
}

Note: this example is not a MediaRecorder usage guide.