Convert utils/textContent.tsx to TS

Summary: Convert utils/textContent.tsx to TS

Reviewed By: danielbuechele

Differential Revision: D16710386

fbshipit-source-id: 225d66965d915fc1c36e382e3c06c7b2dad5d973

src/utils/textContent.tsx

@@ -0,0 +1,45 @@
+/**
+ * Copyright 2018-present Facebook.
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ * @format
+ */
+
+import {ReactNode, ReactElement} from 'react';
+
+function isReactElement(object: any) {
+  return (
+    typeof object === 'object' &&
+    object !== null &&
+    object.$$typeof === Symbol.for('react.element')
+  );
+}
+
+/**
+ * Recursively walks through all children of a React element and returns
+ * the string representation of the leafs concatenated.
+ */
+export default (node: ReactNode): string => {
+  let res = '';
+  const traverse = (node: ReactNode) => {
+    if (typeof node === 'string' || typeof node === 'number') {
+      // this is a leaf, add it to the result string
+      res += node;
+    } else if (Array.isArray(node)) {
+      // traverse all array members and recursively stringify them
+      node.forEach(traverse);
+    } else if (isReactElement(node)) {
+      // node is a react element access its children an recursively stringify them
+      const {children} = (node as ReactElement).props;
+      if (Array.isArray(children)) {
+        children.forEach(traverse);
+      } else {
+        traverse(children);
+      }
+    }
+  };
+
+  traverse(node);
+
+  return res;
+};