const deepMerge = (a, b, fn) =>

  [...new Set([...Object.keys(a), ...Object.keys(b)])].reduce(

    (acc, key) => ({ ...acc, [key]: fn(key, a[key], b[key]) }),

    {}

  );

​

deepMerge(

  { a: true, b: { c: [1, 2, 3] } },

  { a: false, b: { d: [1, 2, 3] } },

  (key, a, b) => (key === 'a' ? a && b : Object.assign({}, a, b))

);

// { a: false, b: { c: [ 1, 2, 3 ], d: [ 1, 2, 3 ] } }

const deepMerge = (a, b, fn) =>

  [...new Set([...Object.keys(a), ...Object.keys(b)])].reduce(

    (acc, key) => ({ ...acc, [key]: fn(key, a[key], b[key]) }),

    {}

  );

​

deepMerge(

  { a: true, b: { c: [1, 2, 3] } },

  { a: false, b: { d: [1, 2, 3] } },

  (key, a, b) => (key === 'a' ? a && b : Object.assign({}, a, b))

);

// { a: false, b: { c: [ 1, 2, 3 ], d: [ 1, 2, 3 ] } }