coinflip.js

function coinflip() {
  let string = "";
  for (var i = 0; i < 20; i++) {
    let flip = Math.round(Math.random());
    string+= flip === 1 ? "H" : "T"
  }
  return string
}

function montecarlo(iters) {
  let numHHHT = 0;
  let numHHHH = 0;

  for (var i = 0; i < iters; i++) {
    let flipResult = coinflip();
    if (flipResult.indexOf("HHHT") !== -1) numHHHT++;
    if (flipResult.indexOf("HHHH") !== -1) numHHHH++;
  }

  console.log("num HHHT = ", numHHHT)
  console.log("num HHHH = ", numHHHH)
}

montecarlo(1000)

There is a bug in line 3. Changing 20 to 4 will fix this