Override the properties of an interface in TypeScript

I know that overriding properties of an interface in an extended interface, modifying their types, is forbidden.

I'm looking for an alternative solution that would allow me to not copy the contents of the first interface (it's pretty big).

Here is below my first naive approach. Given that base interface:

interface OrginalInterface {
    title?: string;
    text?: string;
    anotherProperty?: SomeType;
    // lots of other properties
}

This interface is defined in a library. I can't modify it (ie. add generics, for example) just to satisfy my needs in the extended interface.

In the extended interface, used by a wrapper library (mine), I want to reuse the existing interface, while making some fields having a different type:

interface ExtendedInterface extends OriginalInterface {
    title?: string | ReactElement<any>;
    text?: string | ReactElement<any>;
}

But this is not possible.

error TS2430: Interface 'ExtendedInterface' incorrectly extends interface 'OriginalInterface'.
  Types of property 'title' are incompatible.
    Type 'ReactElement<any>' is not assignable to type 'string'.

I also tried to merge the two interfaces together:

type Extended = OriginalInterface & NewInterfaceWithOverridingPropertiesOnly;

While this passes the compilation, it does not work. If you declare a variable with this type, you'll only be able to assign objects that have a compatible structure with OriginalInterface.

I feel like TypeScript's type-system don't offers me any other way to express my need to declare a new type derived from OrginalInterface. I don't need the new type to be assignable to OriginalInterface ; I just need it to reuse most properties of OriginalInterface.

I'd need something like mapped types with a condition on which properties are affected. Maybe Conditional types from pre-release TypeScript 2.8? Or should I copy the first interface's contents?

UPDATE, 2018-08

TypeScript 2.8 introduced Exclude<T, U> which behaves like the Diff<T, U> defined below for all types (not just key types). You should definitely use Exclude<> instead of Diff<> if you are using TypeScript 2.8 or above.

Also, in TypeScript 2.9, keyof any was expanded from string to string | number | symbol, so the below Diff<T, U> caused errors which can be fixed by changing Diff<T extends string, U extends string> to Diff<T extends keyof any, U extends keyof any>. This change has been made below.

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ORIGINAL ANSWER, 2018-03

Yes, conditional types will enable this, although you can get this behavior without them as well.

The idea is to pull properties out of the original interface and then replace them with new ones. Like this:

type Diff<T extends keyof any, U extends keyof any> = 
  ({ [P in T]: P } & { [P in U]: never } & { [x: string]: never })[T];
type Overwrite<T, U> = Pick<T, Diff<keyof T, keyof U>> & U;

interface OriginalInterface {
  title?: string;
  text?: string;
  anotherProperty?: SomeType;
  // lots of other properties
}
interface Extension {
  title?: string | ReactElement<any>;
  text?: string | ReactElement<any>;
}

interface ExtendedInterface extends Overwrite<OriginalInterface, Extension> {};

const ext: ExtendedInterface = {};  // okay, no required properties
ext.text; // string | ReactElement<any> | undefined
ext.title; // string | ReactElement<any> | undefined
ext.anotherProperty; // SomeType | undefined

EDIT: I changed the definition of Overwrite<T,U> to respect the optional/required status of properties from T whose keys are not present in U.

This has the behavior you want, I think. Hope that helps; good luck!