PhpStorm “Let definition are not supported by current JavaScript version”
Go to Settings / Languages & Frameworks / JavaScript (it may be Preferences / Languages & Frameworks / JavaScript on some OSes) …and set the “JavaScript language version” to “ECMAScript 6” (or, if they’ve updated to reflect current naming, “ECMAScript 2015” — or “ECMAScript 2016” or later if you’re coding to a newer spec):