Io-ts interface for properties with unknown keys

I think the answer you're looking for is record:

const myInterfaceCodec = t.record(t.string, t.union([t.string, t.undefined, t.null]));export type MyInterface = t.TypeOf<typeof myInterfaceCodec>;

=> type MyInterface = { [x: string]: string | null | undefined; }

Your use case:

const myInterfaceV = t.record(t.string, t.union([t.string, t.undefined, t.null]));export type MyInterface = t.TypeOf<typeof myInterfaceV>;const myOtherInterfaceV = t.intersection([    t.type({        requiredProp1: t.string,        requiredProp2: t.string    }),    myInterfaceV]);export type MyOtherInterface = t.TypeOf<typeof myOtherInterfaceV>;const a: MyOtherInterface = {    requiredProp1: "string",    requiredProp2: "string2"};const b: MyOtherInterface = {    requiredProp1: "string",    requiredProp2: "string2",    optionalProp1: "hello",    optionalProp2: "world"};

Probably the most close to myInterface in io-ts is t.UnknownRecord

export const MyOtherInterfaceV = t.interface({  requiredProp1: t.string,  requiredProp2: t.string})const MyOtherInterface = t.intersection([ t.UnknownRecord, MyOtherInterfaceV ]);