How to Add Functions (including useState) to TypeScript Interfaces

Omar Shishani
1 min readSep 10, 2021

I was having trouble finding out how to add functions to TypeScript interfaces. When I finally found out how I decided to write this. I’ll show the format immediatly below:

export default function CardExpirationField({
}: {
showCardExpirationError: boolean,
setValidCardExpiration(arg: boolean): void,
setShowCardExpirationError(arg: boolean): void,
setCardExpirationValue(cardExpirationInput: string): void,
cardExpirationHasBeenBlurred: boolean,
}) {

We can see that to create a type for a function, we need to use the format functionName(nameForArgument: [argument type]): [type] . We need to write the function name with parentheses directly after. Inside the parentheses, we create a name for any arguments passed to the function (separated by comma), and set the expected types of those arguments. We set a colon “:” after the function name and then write the type of the expected output of the function.

Easy once you know how to do it!



Omar Shishani

Hi! I am a React developer who loves technology and learning new things in all different fields.