fit canvas to specific ratio

#target photoshop

main ();

function main ()
{
  if (app.documents.length < 1)
  {
  alert ("No document open to resize.");
  return;
  }

  // These can be changed to create images with different aspect ratios.
  var arHeight = 4;
  var arWidth = 5;

  // Apply the resize to Photoshop's active (selected) document.
  var doc = app.activeDocument;

  // Get the image size in pixels.
  var pixelWidth = new UnitValue (doc.width, doc.width.type);
  var pixelHeight = new UnitValue (doc.height, doc.height.type);
  pixelWidth.convert ('px');
  pixelHeight.convert ('px');

  // Determine the target aspect ratio and the current aspect ratio of the image.
  var targetAr = arWidth / arHeight;
  var sourceAr = pixelWidth / pixelHeight;

  // Start by setting the current dimensions.
  var resizedWidth = pixelWidth;
  var resizedHeight = pixelHeight;

  // The source image aspect ratio determines which dimension, if any, needs to be changed.
  if (sourceAr < targetAr)
  resizedWidth = (arWidth * pixelHeight) / arHeight;
  else
  resizedHeight = (arHeight * pixelWidth) / arWidth;

  // Apply the change to the image.
  doc.resizeCanvas (resizedWidth, resizedHeight, AnchorPosition.MIDDLECENTER);
}