Update README.md

Author: yanglbme

solution/2600-2699/2621.Sleep/README.md

@@ -70,6 +70,23 @@ async function sleep(millis: number): Promise<void> {
  */
 ```
 
+#### JavaScript
+
+```js
+/**
+ * @param {number} millis
+ * @return {Promise}
+ */
+async function sleep(millis) {
+    return new Promise(r => setTimeout(r, millis));
+}
+
+/**
+ * let t = Date.now()
+ * sleep(100).then(() => console.log(Date.now() - t)) // 100
+ */
+```
+
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