Utilizing TypeScript type assertions for interfaces with non-mandatory members

Is there a difference in how interfaces with optional properties are treated compared to those without them? If none of the properties in an interface are defined as optional, are they all considered optional for type assertion?

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

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

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

//This works as expected even if no properties are defined as optional 
let b = { anotherRequiredProperty: '' } as WithoutOptionalProperties;

Answer №1

The reason for this is that type assertions between types A and B will pass if either A can be assigned to B or B can be assigned to A (in simple terms).

In your first case, neither of these conditions holds true. However, in case B, one of these conditions is met (which is why the assertion compiles without errors).

Read more

Learn more about type assertion here

For information on Double Assertion, visit: Double Assertion in TypeScript

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