add deepFreeze

snippets/deepFreeze.md

@@ -0,0 +1,38 @@
+### deepFreeze
+
+Deep freezes an object.
+
+Calls `Object.freeze(obj)` recursively on all unfrozen properties of obj that are `typeof` function or object.
+
+```js
+const deepFreeze = obj => {
+  Object.freeze(obj);
+
+  Object.getOwnPropertyNames(obj).forEach(function(prop) {
+    if (
+      obj.hasOwnProperty(prop) &&
+      obj[prop] !== null &&
+      (typeof obj[prop] === 'object' || typeof obj[prop] === 'function') &&
+      !Object.isFrozen(obj[prop])
+    ) {
+      deepFreeze(obj[prop]);
+    }
+  });
+
+  return obj;
+};
+```
+
+```js
+'use strict';
+
+const a = Object.freeze([1, [2, 3]]);
+
+a[0] = 3; // not allowed
+a[1][0] = 4; // allowed but shouldn't be
+
+const b = deepFreeze([1, [2, 3]]);
+
+b[0] = 3; // not allowed
+b[1][0] = 4; // not allowed as well
+```