Функция deepMerge на TypeScript

deepMerge.ts

type DeepPartial<T> = {
  [P in keyof T]?: DeepPartial<T[P]>
}

function deepMerge<T extends { [key: string]: any }>(
  target: T,
  source: DeepPartial<T>,
): T {
  if (Array.isArray(target)) {
    return target.concat(source) as unknown as T
  }
  if (
    target &&
    source &&
    typeof target === 'object' &&
    typeof source === 'object'
  ) {
    return Object.assign(
      target,
      ...Object.keys(source).map((key) => ({
        [key]: deepMerge(
          target[key as keyof T],
          source[key as keyof T] as DeepPartial<T[keyof T]>,
        ),
      })),
    )
  }
  return source as T
}

Пример

deepMerge({ a: 'a', b: 'b' }, { a: 'a', c: 'c' }) // => {a: "a", b: "b", c: "c"}