feat(keyOf): add keyOf function

src/keyOf.js

@@ -0,0 +1,61 @@
+// o
+import is from './is';
+import empty from './empty';
+import each from './each';
+
+/**
+ * Get the key of the specified value in dot notation
+ *
+ * @param {object} object The object to search
+ * @param {*} value The value to look for
+ * @param {boolean} [follow=false] Whether to follow objects
+ *
+ * @returns {string} The key when found else undefined
+ */
+function keyOf(object, value, follow = false) {
+  // if the object is an object and is not empty
+  if (is(object) && !empty(object)) {
+    // create a found boolean so we can skip
+    // over keys once we have found the correct
+    // key
+    let found = false;
+    // create an result variable as false
+    let result = '';
+
+    // for each key/value in the object
+    // follow is passed into each therefore the
+    // each function works out whether to follow
+    // the objects
+    each(object, (key, objValue) => {
+      // if the result isn't already found
+      if (!found) {
+        // follow is false or follow is true but the
+        // object value isn't an object
+        if (!follow || (follow && !is(value))) {
+          // check if the object value is equal to
+          // the specified value
+          if (objValue === value) {
+            // set found to true since the key was found
+            found = true;
+
+            // if the values are the same set the result
+            // to the key
+            result = key;
+          }
+        }
+      }
+    }, follow);
+
+    // return the result if it was found else return
+    // undefined
+    return found
+      ? result
+      : undefined;
+  }
+
+  // if the object isn't an object or is empty return
+  // false because the object can't be checked
+  return undefined;
+}
+
+export default keyOf;