The start() method of the TimeRanges interface returns the time offset at which a specified time range begins.

index

The range number to return the starting time for.

A number.

IndexSizeError DOMException

Thrown if the specified index doesn't correspond to an existing range.

Given a video element with the ID "myVideo":

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
  }
}

This example looks at the time ranges and looks to see if the entire video has been loaded.