Typescript type statements - an interface with optional members

Question

Typescript type statements - an interface with optional members

Why is an interface with an additional property handled differently than an interface? Are all properties that are considered optional for a type statement if none of them are explicitly defined?

interface WithOptionalProperty {
    requiredProperty: string;
    optionalProperty?: string;
}

//compilation error 'requiredProperty' is missing
let a = { optionalProperty: '' } as WithOptionalProperty; 

interface WithoutOptionalProperties {
    requiredProperty: string;
    anotherRequiredProperty: string;
}

//but this works as expected
let b = { anotherRequiredProperty: '' } as WithoutOptionalProperties;