There are currently three syntaxes that TypeScript allows for function declarations in interfaces:
Using your example of a validation function taking 1 parameter (of any type) and a boolean return value:
validation: {(flag: any): boolean};
or in the newer syntax:
validation(flag: any) : boolean;
or an alternative is:
validation: (flag: any) => boolean;
Solution:
so to make it optional with the old syntax is easy:
validation?: {(flag: any): boolean};
with the second syntax (recent addition – thanks to @toothbrush)
validation?(flag: any) : boolean;
or in the third syntax (as you found):
validation?: (flag: any) => boolean;