splash.js

var use = {
	'iPhone': 'yes',
	'iPhone:retina': 'yes',
	'iPad': 'no',
	'iPad:retina': 'no'
};

/**
 *
 *	iPad:
 *	- Retina: 1536x2048px (portrait) / 2048x1536px (landscape)
 *	- Non-retina: 768x1004px (portrait) / 1024x748px (landscape)
 *
 *	iPhone/iPod:
 *	- iPhone5: 640x1136px
 *	- Retina: 640x960px
 *	- Non-retina: 320x480px
 *
 */

Array.prototype.contains = function (item) {

	for (var n = this.length; n--;) {
		if (item === this[n]) {
			return true;
		}
	}

	return false;

};

String.prototype.contains = function (item) {

	return this.indexOf(item) > -1;

};

// set sizes

var sizes = []; // array-ified object to maintain order

if (doUse('iPad:retina')) {
	sizes.push(
		['iPad_retina', [1536, 2048]],
		['iPad_retina_landscape', [2048, 1536]]
	);
}

if (doUse('iPad')) {
	sizes.push(
		['iPad', [768, 1004]],
		['iPad_landscape', [1024, 748]]
	);
}

if (doUse('iPhone:retina')) {
	sizes.push(
		['iPhone5_retina', [640, 1136]],
		['iPhone_retina', [640, 960]]
	);
}

if (doUse('iPhone')) {
	sizes.push(
		['iPhone', [320, 480]]
	);
}

$.level = 2; // debug

// vars

var doc = app.activeDocument,
	doc_height = parse(doc.height),
	doc_width = parse(doc.width),
	sorted_sizes = [
		[], // landscape_retina
		[], // portrait_retina
		[], // landscape
		[]  // portrait
	],
	group_content = doc.layerSets.getByName('content'),
	size_count = sizes.length,
	original_size,
	saved_sizes = [];

// get largest size, make sure it's large enough

var max_height = 0;
var max_width = 0;

for (var n = size_count; n--;) {

	var dims = sizes[n][1];

	if (dims[0] > max_width) {
		max_width = dims[0];
	}

	if (dims[1] > max_height) {
		max_height = dims[1];
	}

}

original_size = [max_width, max_height];

// check for proper starting document size

if (doc_width !== original_size[0] || doc_height !== original_size[1]) {
	throw new Error('Expected document to be [' + original_size[0] + 'px × ' + original_size[1] + 'px], given [' + doc_width + 'px × ' + doc_height + 'px]');
}

// transform sizes object into portrait and landscape arrays for convenience

for (var j = size_count; j--;) {

	var size = sizes[j];
	var label = size[0];
	var isLandscape = label.contains('landscape');
	var isRetina = label.contains('retina');

	if (isLandscape && isRetina) {

		sorted_sizes[0].push(size); // landscape, retina

	} else if (isRetina) {

		sorted_sizes[1].push(size); // portrait, retina

	} else if (isLandscape) {

		sorted_sizes[2].push(size); // landscape, non-retina

	} else {

		sorted_sizes[3].push(size); // portrait, non-retina

	}
	
}

// save portraits

for (var k = 0, kk = sorted_sizes.length; k < kk; k++) {

	var set = sorted_sizes[k];

	for (var n = set.length; n--;) {

		var size = set[n];

		// resize
		
		if (k === 0 || k === 1) {
			
			// retina
			doc.resizeCanvas(size[1][0], size[1][1], AnchorPosition.MIDDLECENTER);

		} else {

			// non-retina
			doc.resizeImage(size[1][0], size[1][1]);

		}

		// Save for Web options
		opts = new ExportOptionsSaveForWeb();
		opts.format = SaveDocumentType.PNG;
		opts.PNG8 = false;

		// Save it
		file = new File(doc.path + '/' + size[0] + '.png');
		doc.exportDocument(file, ExportType.SAVEFORWEB, opts);

		saved_sizes.push(size[0]);

	}
}

alert('Done! Generated splash screens for ' + saved_sizes.join(', '));

// helpers

function parse (string) {
	return parseInt(string, 10);
}

function doUse (what) {
	return use[what] && use[what] === 'yes';
}