how to find area of polygon with coordinates in javascript

Algorithm to find the area of a polygon
If you know the coordinates of the vertices of a polygon, this algorithm can be used to find the area.

Parameters
X, Y	Arrays of the x and y coordinates of the vertices, traced in a clockwise direction, starting at any vertex. If you trace them counterclockwise, the result will be correct but have a negative sign.
numPoints	The number of vertices
Returns	the area of the polygon
The algorithm, in JavaScript:

function polygonArea(X, Y, numPoints) 
{ 
area = 0;   // Accumulates area 
j = numPoints-1; 

for (i=0; i<numPoints; i++)
{ area +=  (X[j]+X[i]) * (Y[j]-Y[i]); 
  j = i;  //j is previous vertex to i
}
  return area/2;
}