parallaxing purchase window in Javascript for Article Preview Prototype

parallax-purchasewindow.js

// positionNow is the current X offset of the vertical scrollable frame
purchaseWindow.originalX: (screenWidth - purchaseWindow.width)/2 // centered position of purchase Window


  // parallax purchaseWindow
	parallaxPurchaseWindow = function(min,visibility) {
				// visibility (true/false) determines if we offset it by screenWidth;
				if (visibility)
					offset = screenWidth;
				else
					offset = 0;
				purchaseWindowX = ((positionNow - min)*-1) + offset;
				
				// https://medium.com/building-potluck/2e405d50b600
				parallaxOffset = Math.round(utils.map_range(purchaseWindowX, -2048, 2048, purchaseWindow.originalX, purchaseWindow.originalX*-1));
		
				purchaseWindow.x = purchaseWindowX + purchaseWindow.originalX + parallaxOffset;
				console.log("purchaseWindowX: "+purchaseWindowX+" ~ parallaxOffset: "+parallaxOffset);
	};

	utils.map_range = function(value, low1, high1, low2, high2) {
	 if (value < low1) { return low2; }
	 else if (value > high1) { return low2; }
	 else return low2 + (high2 - low2) * (value - low1) / (high1 - low1);
	}