Creating an API of modifying a JSON object. Need help with Inheritance.

TYPESCRIPT: I have the majority api finished. I need to modify the api to work with my new inheritance. I can walk you through but the inheritance is throwing me off. It's a small project, here is the inheritance.
export interface ITestMetaDataResult {
success?: {
[filename: string]: ITestMetaDataTestSuiteP4
}
failure?: {
filename: string[]
next_step: string[]
}
}