The cause:

This error happens because you are calling the function myFunction(10), so the value of b is not defined.

Solution:

To solve this error, you need to give a default value to b as bellow:

function myFunction(a: number, b: number = 20): number {
     return (a * b);
}

If you are creating a component inside an Angular application, you also get this error. You can supply a default value during property declaration or assign a value inside the constructor.

To know more solutions for the error: ”object is possibly ‘undefined’”, you can read this helpful post: https://ittutoria.net/resolve-error-in-typescript-object-is-possibly-undefined/