How to add cell border to SheetJS .xlsx generated file?

You are using community version of SheetJS library and with this version can not do styling. BUT in Pro Version styling feature is available.

Checkout this official comment:

This is the community version. We also offer a pro version with performance enhancements, additional features like styling, and dedicated support.

For more information about Pro Version visit their official site: http://sheetjs.com/pro

var wscols = [{wpx: 100}];
ws['!cols'] = wscols ;