Typescript type assertions - interface with optional members

Question

Why interface with optional property is treated differently from interface without? Are all the properties considered optional for type assertion if none of them defined optional explicitly?

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;

Answer 1

This is because type assertions between types A and B succeed if A is assignable to B or B is assignable to A (simplified explanation).

Neither of these conditions are true in your case 1. But one of these is true in case B (hence the assertion compiles fine).

More

https://basarat.gitbooks.io/typescript/content/docs/types/type-assertion.html

Double Assertion : https://basarat.gitbooks.io/typescript/content/docs/types/type-assertion.html#double-assertion