var element = document.getElementById('id');
var options = {
  grid: 10,
  onDrag: function(){ ... }
};
new Draggable (element, options);

var Draggable = require ('Draggable');
var element = document.getElementById('id');
new Draggable (element);

// no limit
limit: null

// limit x, but leave y unbounded
limit: {
  x: [1,10],
  y: null
}

// limit both axes
limit: {
  x: [1,10],
  y: [1,500]
}

// bound x, set y to a constant
limit: {
  x: [1,10],
  y: 5
}

// bound with an element
limit: document.getElementById('id')

// bound with a custom function
limit: function (
  x,  // current X coordinate
  y,  // current Y coordinate
  x0, // original X coordinate (where drag was started)
  y0  // original Y coordinate (where drag was started)
) {

  var radius = 100,
    dx = x - x0,
    dy = y - y0,
    distance = Math.sqrt(dx*dx + dy*dy),

    // only allow dragging within a circle of radius 100
    outOfRange = distance > radius;


  // if our point is outside of the circle, compute the
  // point on the circle's edge closest to our point
  if (outOfRange) {

    x = x0 + radius * (x - x0) / distance;
    y = y0 + radius * (y - y0) / distance;

  }

  return {
    x: x,
    y: y
  };

}