The snapToLines property of the VTTCue interface is a Boolean indicating if the VTTCue.line property is an integer number of lines, or a percentage of the video size.

A Boolean.

In the following example a new VTTCue is created, then the value of snapToLines is set to true. The value is then printed to the console.

let video = document.querySelector("video");
let track = video.addTextTrack("captions", "Captions", "en");
track.mode = "showing";

let cue1 = new VTTCue(0, 0.9, "Hildy!");
cue1.snapToLines = true;
console.log(cue1.snapToLines);

track.addCue(cue1);