Useful typescript interfaces

Generic Interfaces.ts

/**
 * Removes ReadOnly from all properties on an interface.
 */
type Mutable<T> = {
	-readonly [P in keyof T]: T[P] extends ReadonlyArray<infer U> ? Mutable<U>[] : Mutable<T[P]>
};

MapObject.ts

/**
 * A per-item typed `Map` object.
 * @author /r/mcaruso
 * @see https://www.reddit.com/r/typescript/comments/dn2xzd/typing_a_map/
 */
interface MapObject<Props extends object> extends Map<keyof Props, Props[keyof Props]> {
	get<K extends keyof Props>(key : K) : Props[K];
	set<K extends keyof Props>(key : K, val: Props[K]): this;
	forEach<K extends keyof Props>(callbackfn: (value: Props[K], key: K, map: any) => void, thisArg?: any): void;
	has<K extends keyof Props>(key: K): boolean;
	delete<K extends keyof Props>(key: K): boolean;
}

/**
 * How to use:
 */
// Force type:
const map = new Map() as MapObject<{ imgur: ImageData; twitter: string; }>;	
// Works:
map.set('twitter', 'final');	
// Error as expected:
map.set('imgur', 3);	
// Returns a ImageData:
var s = map.get('imgur');	
// Returns a string:
var t = map.get('twitter');	
map.forEach((value, key, map) => {
	// Unfortunately I'm unable to find a way for foreach to reduce the type on asserting the key content:
	if(key === 'twitter') {
		var f = value;	// string | ImageData
	}
});