Well, there once was a forlorn EcmaScript version, aka ES4, which included rather similar concepts ( http://www.ecmascript.org/es4/spec/overview.pdf http://www.

Well, there once was a forlorn EcmaScript version, aka ES4, which included rather similar concepts ( http://www.ecmascript.org/es4/spec/overview.pdf http://www.ecmascript.org/es4/spec/overview.pdf ) : > Structural function types describe functions and methods. The type > function (int, string): boolean > describes a Function object that takes two arguments, one int and the other string, and which returns a boolean Given that a great many other concepts from ES4 are finding their ways into ES6, it is not implausible that this one may also reappear into a more consensual specification someday.