deepMerge

Deeply merges two objects, using a function to handle keys present in both.

• Use Object.keys() to get the keys of both objects, create a Set from them and use the spread operator (...) to create an array of all the unique keys.
• Use Array.prototype.reduce() to add each unique key to the object, using fn to combine the values of the two given objects.

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 ] } }