WebStorm “Let definition are not supported by current JavaScript version” [duplicate]
It can be set up like in the following image:
Flow: File -> Settings -> Languages & Frameworks -> Javascript
Make sure you restart your IDE after changing the JS language version to ECMAScript 6.
You need to set your JavaScript version in the IDE. This can be accessed on Windows with ctrl + alt + s. You need at least ECMA Script 6 to use let.