diff --git a/js/freq.ts b/js/freq.ts
index bd90f49..cc34b51 100644
--- a/js/freq.ts
+++ b/js/freq.ts
@@ -17,3 +17,21 @@ Math.ceil(.95) // 1
 // 给定数字的值四舍五入到最接近的整数。
 Math.round(20.49);   //20
 Math.round(20.5);    //21
+
+
+// For iterating on keys of Arrays, Strings, or Objects, use for .. in :
+for (let key in yourobject) {
+  console.log(key, yourobject[key]);
+}
+
+// With ES6, if you need both keys and values simultaneously, do
+for (let [key, value] of Object.entries(yourobject)) {
+    console.log(key, value);
+}
+
+// To avoid logging inherited properties, check with hasOwnProperty :
+for (let key in yourobject) {
+   if (yourobject.hasOwnProperty(key)) {
+      console.log(key, yourobject[key]);
+   }
+}