In release 2.0 (with typescript) "is" functions are not custom guard functions

[sp3ber]

It's possible to create custom type guard functions in typescript - https://basarat.gitbooks.io/typescript/docs/types/typeGuard.html

For example, function '/is/array' could be some something like this
const isArray = function (test): test is string { return !!test && Array.isArray(test); };