Add isDeepFrozen

snippets/isDeepFrozen.md

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+---
+title: isDeepFrozen
+tags: object,recursion,intermediate
+---
+
+Checks if an object is deeply frozen
+
+Use recursion.
+Use `Object.isFrozen()` on the given object.
+Use `Object.keys()`, `Array.prototype.every()` to check that all keys are either deeply frozen objects or non-object values.
+
+```js
+const isDeepFrozen = obj =>
+  Object.isFrozen(obj) &&
+  Object.keys(obj).every(
+    prop => typeof obj[prop] !== 'object' || isDeepFrozen(obj[prop])
+  );
+```
+
+```js
+const x = Object.freeze({ a: 1 });
+const y = Object.freeze({ b: { c: 2 } });
+isDeepFrozen(x); // true
+isDeepFrozen(y); // false
+```