How can I extend an interface and override a property’s type?

I have a pretty complicated object with multiple properties that I’d like to extend and override a specific property.

interface ComplicatedObject {
  propertyOne: string,
  propertyTwo: null
}

interface MoreComplicatedObject extends ComplicatedObject {
  propertyTwo: string
}

Essentially, objects with the type ComplicatedObject are converted to the MoreComplicatedType by assigning a string value to propertyTwo. I’d like to avoid using a union type on propertyTwo because all calls using propertyTwo assume that it’s a string, not a null value, so I’d rather not have to include type checks in every instance where I access propertyTwo.

How can I extend an interface and override the type of an existing property?

Answer

You can override property type when extending an interface only if the type in the extending interface is compatible with original type of the property. Usual case is when you are overriding it with more restrictive type.

It means that you have to have foresight and declare original property with a type that will be compatible with all possible extensions. In your case, you can use union type in ComplicatedObject:

interface ComplicatedObject {
  propertyOne: string,
  propertyTwo: null | string
}

interface MoreComplicatedObject extends ComplicatedObject {
  propertyTwo: string
}

Or you can make CompicatedObject generic as described in this answer.