TypeScript custom declaration files for untyped npm modules

The declaration declare module 'shiitake'; should be in a global scope. i.e. a top-level declaration in a non-module (where a module is a file with at least one top-level import or export).

A declaration of the form declare module '...' { } in a module is an augmentation. For more details see Typescript Module Augmentation.

So you want this file to look like this:

declare module 'shiitake' {

    import * as React from 'react';

    export interface ShiitakeProps {
        lines: number;
    }

    export default class Shiitake extends React.Component<ShiitakeProps, any> { 
    }
}