In my opinion one of the most useful features is first-class support for algebraic data types. Using records and variants ('sum' and 'product' types) in Reason

In my opinion one of the most useful features is first-class support for algebraic data types. Using records and variants ('sum' and 'product' types) in Reason one can be much more expressive in terms of how program state and other constraints are defined. One can get some of the same benefits of sum types in TypeScript using discriminated unions - but the syntax is cumbersome and requires boilerplate as compared to Reason.