Update README.md

Author: yanglbme

solution/0500-0599/0564.Find the Closest Palindrome/README.md

@@ -189,6 +189,61 @@ func abs(x int) int {
 }
 ```
 
+#### JavaScript
+
+```js
+/**
+ * @param {string} n
+ * @return {string}
+ */
+
+function nearestPalindromic(n) {
+    const x = BigInt(n);
+    let ans = null;
+
+    for (const t of getCandidates(n)) {
+        if (
+            ans === null ||
+            absDiff(t, x) < absDiff(ans, x) ||
+            (absDiff(t, x) === absDiff(ans, x) && t < ans)
+        ) {
+            ans = t;
+        }
+    }
+
+    return ans.toString();
+}
+
+function getCandidates(n) {
+    const length = n.length;
+    const res = new Set();
+
+    res.add(BigInt(Math.pow(10, length - 1) - 1));
+    res.add(BigInt(Math.pow(10, length) + 1));
+
+    const left = BigInt(n.substring(0, Math.ceil(length / 2)));
+
+    for (let i = left - 1n; i <= left + 1n; i++) {
+        const prefix = i.toString();
+        const t =
+            prefix +
+            prefix
+                .split('')
+                .reverse()
+                .slice(length % 2)
+                .join('');
+        res.add(BigInt(t));
+    }
+
+    res.delete(BigInt(n));
+    return res;
+}
+
+function absDiff(a, b) {
+    return a > b ? a - b : b - a;
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->