Calculate current position according to start point destination point speed and time

geometry.js

// calculate a new point according to start point, destination, speed and time.
(function(root) {
  
  // start and destination are points given as arrays of length 2
  // speed is in m/s
  // time is in seconds
  var currentPosition = function(start, destination, speed, time) {
    var distanceTravelled = speed * time;
    
    // calulate the diff x and diff y to be travelled
    var diffx = destination[0] - start[0];
    var diffy = destination[1] - start[1];
    
    // calculate the direction in degrees
    var degrees;
    if(diffx === 0) {
      if(diffy < 0) {
        // just deturn start point with the distance travelled reduced from y
        start[1] -= distanceTravelled;
        return start;
      } else {
        start[1] += distanceTravelled;
        return start;
      }
    } else {
      degrees = Math.atan(diffy / diffx);
    }
    
    // calculate the x distance travelled
    var xTravelled = Math.sin(degrees) * distanceTravelled;
    var yTravelled = Math.cos(degrees) * distanceTravelled;
    
    return [xTravelled, yTravelled];
  };
  
  root.currentPosition = currentPosition;
})(this);