Edit in JSFiddle

var page = require('webpage').create();

var system = require('system');
var args = system.args;

var url = args[1]
var viewPortSizeOfDevice = args[2]
var fileName = args[3]

console.log(viewPortSizeOfDevice);

if (viewPortSizeOfDevice === "desktop") {
	page.viewportSize = {
		width: 1319,
		height: 222
	}
};

if (viewPortSizeOfDevice === "mobile") {
	page.viewportSize = {
		width: 375,
		height: 667
		}
};

page.open(url, function(status) {
 console.log("Status: " + status);
 if(status === "success") {
   page.render(fileName + '.png');
 }
 phantom.exit();
});