MediaRecorder: start() method

The start() method of the MediaRecorder interface begins recording media into one or more Blob objects.

You can record the entire duration of the media into a single Blob (or until you call requestData()), or you can specify the number of milliseconds to record at a time. Then, each time that amount of media has been recorded, an event will be delivered to let you act upon the recorded media, while a new Blob is created to record the next slice of the media.

Assuming the MediaRecorder's state is inactive, start() sets the state to recording, then begins capturing media from the input stream. A Blob is created and the data is collected in it until the time slice period elapses or the source media ends. Each time a Blob is filled up to that point (the timeslice duration or the end-of-media, if no slice duration was provided), a dataavailable event is sent to the MediaRecorder with the recorded data. If the source is still playing, a new Blob is created and recording continues into that, and so forth.

When the source stream ends, state is set to inactive and data gathering stops. A final dataavailable event is sent to the MediaRecorder, followed by a stop event.

Note: If the browser is unable to start recording or continue recording, it will raise an error event, followed by a dataavailable event containing the Blob it has gathered, followed by the stop event.




timeslice Optional

The number of milliseconds to record into each Blob. If this parameter isn't included, the entire media duration is recorded into a single Blob unless the requestData() method is called to obtain the Blob and trigger the creation of a new Blob into which the media continues to be recorded.

Return value

None (undefined).


Errors that can be detected immediately are thrown as DOM exceptions. All other errors are reported through error events sent to the MediaRecorder object. You can implement the onerror event handler to respond to these errors.

InvalidStateError DOMException

Thrown if the MediaRecorder is not in the inactive state; you cannot start recording media if it is already being recorded. See the state property.

NotSupportedError DOMException

Thrown if:

  • The media stream you are attempting to record is inactive.
  • One or more of the stream's tracks is in a format that cannot be recorded using the current configuration
  • The videoKeyFrameIntervalDuration and videoKeyFrameIntervalCount parameter are both specificed when creating the MediaRecorder.
SecurityError DOMException

Thrown if the MediaStream is configured to disallow recording. This may be the case, for example, with sources obtained using getUserMedia() when the user denies permission to use an input device. This exception may also be delivered as an error event if the security options for the source media change after recording begins.


record.onclick = () => {
  console.log("recorder started");


MediaStream Recording
# dom-mediarecorder-start

Browser compatibility

BCD tables only load in the browser

See also