The preview release announcement for this version made FP on HN already, but it's a biggie:

* Optional Chaining & Coalescing

* Assertion Functions

* .d.ts Emit From .js Files

* Smarter Control Flow Analysis

* Flatter Error Messages

also great:

* Function Truthy Checks / Uncalled Function Checks (which was tslint's biggest value prop for me up until now)