how to find area of polygon with coordinates in javascript

1Algorithm to find the area of a polygon
2If you know the coordinates of the vertices of a polygon, this algorithm can be used to find the area.
3
4Parameters
5X, 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.
6numPoints	The number of vertices
7Returns	the area of the polygon
8The algorithm, in JavaScript:
9
10function polygonArea(X, Y, numPoints) 
11{ 
12area = 0;   // Accumulates area 
13j = numPoints-1; 
14
15for (i=0; i<numPoints; i++)
16{ area +=  (X[j]+X[i]) * (Y[j]-Y[i]); 
17  j = i;  //j is previous vertex to i
18}
19  return area/2;
20}
21
22