Javascript - Function to calculate polygon area, Algorithm to calculate polygon area

Function to calculate polygon area

Algorithm to calculate polygon area in Javascript

 function polygonArea(X, Y, numPoints) 
 { 
 area = 0; // Accumulates area in the loop
 j = numPoints-1; // The last vertex is the 'previous' one to the first
 
 for (i=0; i<numPoints; i++)
 { area = area + (X[j]+X[i]) * (Y[j]-Y[i]); 
  j = i; //j is previous vertex to i
 }
  return area/2;
 }

Where,

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

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