A TypeScript type used to union together the keys of two or more objects.

merged-union-type.ts

type InternalMergedUnionType<T0 extends {}, T1 extends {}> = {
	
	[K in keyof (T0 & T1)]: K extends keyof T0 ?
		(K extends keyof T1 ?
			T0[K] | T1[K] :
			T0[K]) :
		(K extends keyof T1 ?
			T1[K] :
			never);
	
};

type MergedUnionType<
	T0 extends {},
	T1 extends {},
	T2 extends {} = {},
	T3 extends {} = {},
	T4 extends {} = {},
	T5 extends {} = {},
	T6 extends {} = {},
	T7 extends {} = {},
	T8 extends {} = {},
	T9 extends {} = {}> =
	InternalMergedUnionType<
		InternalMergedUnionType<
			InternalMergedUnionType<
				InternalMergedUnionType<
					InternalMergedUnionType<
						InternalMergedUnionType<
							InternalMergedUnionType<
								InternalMergedUnionType<
									InternalMergedUnionType<T0, T1>, T2
									>, T3
								>, T4
							>, T5
						>, T6
					>, T7
				>, T8
			>, T9
		>;

type MyType1 = {
	
	firstName: string,
	lastName: string
	
};

type MyType2 = {
	
	age: number,
	alive: boolean,
	firstName: null
	
};

type MyType3 = {
	
	lastName?: number,
	alive: boolean
	
};

declare let result: MergedUnionType<MyType1, MyType2, MyType3>;

result.firstName; // string | null
result.lastName;  // string | number | undefined
result.age;       // number
result.alive;     // boolean