Javascript 2d spatial hash

HashMap.js

var HashMap = function(cell_size) {
 this.cell_size = cell_size;
 this.grid = [];
}

HashMap.prototype._key = function(vec) {
  var cellsize = this.cell_size;
  return Math.floor(vec.x/cellsize) * cellsize + ' ' +
         Math.floor(vec.y/cellsize) * cellsize;
}

HashMap.prototype.insert = function(ob) {
  var obkey = this._key(ob.position);
  var grid = this.grid;

  if (!grid[obkey]) {
    grid[obkey] = [];
  }  

  grid[obkey].push(ob);
}

HashMap.prototype.getClosest = function(ob) {
  return this.grid[this._key(ob.position)];
}

Grid should be an object, not an array.