Page size to screen size conversion

pageToScreen.js

 const dpi = 72;            // dots per inch (ie. resolution)
 const factor = 2.54;       // convert inches to cm
 const dpcm = dpi / factor; // dots per cm
 
/**
 * Calculate the phantom browser's paper size property based on the
 * desired size of the export document
 * @param  {Number} [widthIn]     Width in inches of phantom export doc
 * @param  {Number} [heightIn]    Height in inches of phantom export doc
 * @param  {String} [orientation] Paper orientation of phantom export doc
 * @return {Object}               Paper size property for phantom browser
 */

exports.getPaperSize = (widthIn, heightIn, orientation) => {
  orientation = orientation || 'portrait';
  widthIn = widthIn || orientation === 'portrait' ? 8.5 : 11;
  heightIn = heightIn || orientation === 'portrait' ? 11 : 8.5;

  let widthCm = widthIn * factor,
      heightCm = heightIn * factor;

  return {
    width: Math.round(widthCm * dpcm) + 'px',
    height: Math.round(heightCm * dpcm) + 'px',
    orientation: 'portrait',
    margin: '1cm'
  };
};