TypeScript signature of Promise#catch onrejected handler -

i notice standard typing promise#catch in es6-shim.d.ts defined as

interface promise<t> {   catch(onrejected?: (reason: any) => t | promiselike<t>): promise<t>;   catch(onrejected?: (reason: any) => void): promise<t>; } 

i wondering why written way, instead of as

catch(onrejected?: (reason: any) => t | promiselike<t> | void): promise<t>; 

some other file es6-promise.d.ts nate archibald gives single signature of

catch<u>(onrejected?: (error: any) => u | thenable<u>): promise<u>; 

not referring <t> @ all.

so second overloaded signature in es6.shim-d-ts, far know in heavy use in compile-to-es5 environment, unnecessary/undesirable?

well, t can void means use case supported.

if did t | promiselike<t> | void output might have been promise<string> that's resolved void type error.

note promises aren't statically typed in strong way in these definitions. signature harder since reason typed any. languages (like c#) sport unchecked exceptions , unlike java , swift feature checked exceptions (a part of signature of function might throw).