Declaring const of generic type

Question

Attempting to reduce boilerplate, I'm declaring some sort of generic function interface as a type. Then I want to declare a const of such type. So, why typescript assumes that foo declaration is legit and bar is not? Aren't these declarations practically identical? Is typescript lacking simple feature or am I missing some details? Is there any workarounds, if I do not want explicitly repeat FunctionType interface?

type FunctionType<TValue> = (value: TValue) => void;  const foo = <TValue>(value: TValue): void => { }  //const bar: FunctionType<TValue> = (value) => { // Cannot find name 'TValue' //} 

Answer 1

There is a difference between a generic type that happens to be a function and a type that is a generic function.

What you defined there is a generic type that is a function. This means that we can assign this to consts that have the generic types specified:

type FunctionType<TValue> = (value: TValue) => void; const bar: FunctionType<number> = (value) => { // value is number } 

To define a type that is a generic function we need to put the type parameter before the arguments list

type FunctionType = <TValue>(value: TValue) => void; const bar: FunctionType = <TValue>(value) => { // generic function }