The MediaTrackSupportedConstraints dictionary's autoGainControl property is a read-only Boolean value which is present (and set to true) in the object returned by MediaDevices.getSupportedConstraints() if and only if the user agent supports the autoGainControl constraint. If the constraint isn't supported, it's not included in the list, so this value will never be false.

You can access the supported constraints dictionary by calling navigator.mediaDevices.getSupportedConstraints().

The autoGainControl constraint indicates whether or not the browser offers the ability to automatically control the gain (volume) on media tracks; this obviously is contingent on whether or not the individual device supports automatic gain control as well; it's typically a feature provided by microphones.

This property is present in the dictionary (and its value is always true) if the user agent supports the autoGainControl constraint. If the property isn't present, this property is missing from the supported constraints dictionary, and you'll get undefined if you try to look at its value.

This example displays whether or not your browser supports the autoGainControl constraint.

const result = document.getElementById("result");
const supported =
  navigator.mediaDevices.getSupportedConstraints().autoGainControl;
result.textContent = supported ? "Supported!" : "Not supported!";

• Media Capture and Streams API
• MediaDevices.getSupportedConstraints()
• MediaTrackSupportedConstraints
• MediaStreamTrack