Anton Selin Anton Selin - 1 month ago 11
Javascript Question

TypeScript Optional function in Interface

Is it possible to create an Interface in TypeScript with optional function?

interface IElement {
name: string;
options: any;
type: string;
value?: string;
validation(any): boolean; --> should be optional.

}

Answer

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;