speedometer.js

// Thanks! https://github.com/mafintosh/speedometer
var tick = 1;
var maxTick = 65535;
var resolution = 4;
var inc = function() {
  tick = (tick + 1) & maxTick;
};

var timer = setInterval(inc, (1000 / resolution) | 0);
if (timer.unref) timer.unref();

export default function speedometer(seconds) {
  var size = resolution * (seconds || 5);
  var buffer = [0];
  var pointer = 1;
  var last = (tick - 1) & maxTick;

  return function(delta) {
    var dist = (tick - last) & maxTick;
    if (dist > size) dist = size;
    last = tick;

    while (dist--) {
      if (pointer === size) pointer = 0;
      buffer[pointer] = buffer[pointer === 0 ? size - 1 : pointer - 1];
      pointer++;
    }

    if (delta) buffer[pointer - 1] += delta;

    var top = buffer[pointer - 1];
    var btm = buffer.length < size ? 0 : buffer[pointer === size ? 0 : pointer];

    return buffer.length < resolution
      ? top
      : (top - btm) * resolution / buffer.length;
  };
}