ES2015 TCO is in 5.1 but hidden behind a flag. We are preferring the explicit approach championed by Mozilla and Microsoft (<a href="https:&#x2F;&#x2F;github.com&#x2F;tc39&#x2F;proposal-ptc-syntax" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;tc39&#x2F;proposal-ptc-syntax</a>) because it makes the debugging and error analysis story better.