ant path with Mapbox-gl-js https://jsfiddle.net/cs09g/vtmLd6g9/

antpath.js

let dash = [3, 3]; // your input; dash you want to animate
let speed = 0.5; // your input; speed rate
let dashArraySeq = createDashArraySeq(dash, speed); // `dashArraySeq` contains dash arrays you can use

function createDashArraySeq(dash, speed = 1) {
  let dashArraySeq = [dash];
  for (let i = speed, len = dash[0] + dash[1]; i < len; i += speed) {
    const arr = [];
    if (i <= len - dash[0]) {
      arr.push(0, i, dash[0], dash[1] - i);
    } else {
      const leftFillCnt = i - (len - dash[0]);
      arr.push(leftFillCnt, dash[1], dash[0] - leftFillCnt, 0);
    }
    dashArraySeq.push(arr);
  }
  return dashArraySeq;
}

let dashArrayIdx = 0;
function animateLine(ts) {
  dashArrayIdx = (dashArrayIdx + 1) % dashArraySeq.length;
  map.setPaintProperty('line-layer', 'line-dasharray', dashArraySeq[dashArrayIdx]);
  requestAnimationFrame(animateLine);
}

requestAnimationFrame(animateLine);