Assign with type checking (typescript)

assign.ts

type NonFunctionPropertyNames<T> = { [K in keyof T]: T[K] extends Function ? never : K }[keyof T];
type NonFunctionProperties<T> = Pick<T, NonFunctionPropertyNames<T>>;

type Omit<T, K extends keyof T> = Pick<T, ({ [P in keyof T]: P } & { [P in K]: never } & { [x: string]: never })[keyof T]>;

export function assign<T extends NonFunctionProperties<Omit<S, K>>, S extends Object, K extends keyof S>(target: T, source: S, exclude?: K[]): void {
  Object.keys(source).forEach(key => {

    // exclude Object methods
    if (typeof source[key] === 'function') {
      return;
    }

    // exclude excluded Object keys
    if ((exclude || []).indexOf(key) >= 0) {
      return;
    }
    target[key] = source[key];
  })
}