FunctionMerge.md

typelab / utils / FunctionMerge

type FunctionMerge<Target, Source, Z> = [Target, Source] extends [Fn<infer TargetParams, infer TargetReturn>, Fn<infer SourceParams, infer SourceReturn>] ? (...param) => ObjectMerge<TargetReturn, SourceReturn, Z> : never;

Merges the parameter and return types of Source into Target.

Use `ArrayMerge` for the parameter type and `ObjectMerge` for the return type.

• If Target or Source is not Function, it returns never.

Type Parameters

Type Parameter	Default type	Description
Target extends Fn	‐	The target Function.
Source extends Fn	‐	The source Function to merge from.
Z extends _LookupType	"shallow"	‐

Returns

A new Function combining parameter and return types from both Target and Source.

Example

type Obj1 = { a: string; b: { a: string } };
type Obj2 = { a: number; b: { b: number } };

// (param_0: string | number, param_1: string) => { a: string | number; b: { a: string; } | { b: number; }; }
type Shallow = FunctionMerge<(...param: [string, string]) => Obj1, (...param: [number]) => Obj2, 'shallow'>;

// (param_0: string | number, param_1: string) => { a: string | number; b: { a: string; b: number; }; }
type Deep = FunctionMerge<(...param: [string, string]) => Obj1, (...param: [number]) => Obj2, 'deep'>;