script to change the canvas size to square, making the extra space black

square-it.js

doc = app.activeDocument;  

// change the color mode to RGB.  Important for resizing GIFs with indexed colors, to get better results
doc.changeMode(ChangeMode.RGB);  

// Makes the default background white
var black = new SolidColor(); 
white.rgb.hexValue = "000000";
app.backgroundColor = black;

// new size
var newHeightWidth = Math.max(doc.height,doc.width)

// Convert the canvas size as informed above for the END RESULT
app.activeDocument.resizeCanvas(UnitValue(newHeightWidth,"px"),UnitValue(newHeightWidth,"px"));

// our web export options
var options = new ExportOptionsSaveForWeb();
options.quality = 70;
options.format = SaveDocumentType.JPEG;
options.optimized = true;

var newName = 'web-'+doc.name+'.jpg';

doc.exportDocument(File(doc.path+'/'+newName),ExportType.SAVEFORWEB,options);