AudioRecorder

Record microphone audio to a Blob, working around a real browser bug rather than just wrapping MediaRecorder.

Chrome (and Chromium-based browsers) writes WEBM files from MediaRecorder without duration metadata: the file plays, but seeking and showing a duration fail until the whole blob has been decoded once. AudioRecorder requests the microphone, records, and on stop() patches the WEBM container's duration field in place before handing back the Blob, so the recording behaves like a normal audio file immediately.

Recording bytes and recognizing speech are separate concerns. See createSpeechRecognizer if what you actually want is a transcript, not an audio file.

Usage

import { AudioRecorder } from 'ranuts/utils';

const recorder = new AudioRecorder(); // requests microphone access immediately

startButton.addEventListener('click', () => recorder.start());
pauseButton.addEventListener('click', () => recorder.pause());

stopButton.addEventListener('click', () => {
  const blob = recorder.stop();
  if (blob) audioEl.src = URL.createObjectURL(blob);
});

API

new AudioRecorder()

Requests getUserMedia({ audio: true }) as soon as it's constructed, and begins recording once permission is granted. There is no separate "arm" step: construction is the permission prompt.

start()

Resumes recording if it was pause()d. Returns the underlying MediaRecorder, or undefined before the microphone stream is ready.

pause()

Pauses an active recording. Returns the underlying MediaRecorder, or undefined.

stop()

Stops recording and returns the recorded Blob synchronously, before the container fix (fixDuration) has actually run. Patching the duration needs the full buffer, which is only available once the underlying MediaRecorder's stop event has fired. In practice this only matters if you read recorder.stop()'s return value at the exact instant it resolves versus a moment later; awaiting the next microtask, or reading recorder.blob after the dataavailable/stop sequence settles, is the safe order.

Notes

1. One recorder, one stream. There's no destroy()/teardown; the microphone track stays open for the component's lifetime once granted. Don't construct a new AudioRecorder per recording; reuse one and call start()/stop().
2. Permission is requested at construction, not at start(). If you want to defer the browser's permission prompt until the user actually clicks record, defer constructing AudioRecorder itself, not just the call to start().
3. The duration fix only touches audio/webm. Other MIME types the browser might pick (audio/mp4, audio/ogg, audio/wav, audio/aac) are returned as-is.
4. Errors during getUserMedia (denied permission, no microphone) are logged to the console rather than thrown or surfaced through a callback. Check for a real MediaDevices prompt in your own UI rather than relying on this class to report denial.