Tip: SuiteScript 2.1 = ECMA6+ in client scripts. It is frequently desirable to use post-2015 JavaScript in the context of a client script, yet it is presently disallowed. (The stated reason for disallowing SS2.1 in client scripts, browser support, is spurious and regressive.) No such restriction exists on custom modules, and you can in fact include a SS2.1 module in a client script. So, if the use of ECMA6+ in the context of a client script would prove useful to you, place what you need in a SS2.1 custom module and require it in the define portion of your SS2.x client script.