extent.js

var topLeftX = proj(mapDim.topLine[0])[0];
var topLeftY = proj(mapDim.topLine[0])[1];
var topRightX = proj(mapDim.topLine[mapDim.topLine.length - 1])[0];
var bottomLeftY = proj(mapDim.bottomLine[mapDim.topLine.length - 1])[1];

var extentWidth = topRightX - topLeftX;
var extentHeight = bottomLeftY - topLeftY;

this.extentWidth = extentWidth;
this.extentHeight = extentHeight;