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+---
+title: deepMerge
+tags: object,function,advanced
+firstSeen: 2021-07-25T05:00:00-04:00
+---
+
+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.
+
+```js
+const deepMerge = (a, b, fn) =>
+  [...new Set([...Object.keys(a), ...Object.keys(b)])].reduce(
+    (acc, key) => ({ ...acc, [key]: fn(key, a[key], b[key]) }),
+    {}
+  );
+```
+
+```js
+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 ] } }
+```