TimeRanges: свойство length

const v = document.getElementById("myVideo");

const buf = v.buffered;

const numRanges = buf.length;

if (buf.length === 1) {
  // Only one range
  if (buf.start(0) === 0 && buf.end(0) === v.duration) {
    // The one range starts at the beginning and ends at
    // the end of the video, so the whole thing is loaded
  }
}