diff --git a/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/README.md b/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/README.md
index b41b40aa0673f..aeb12cc1b0568 100644
--- a/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/README.md	
+++ b/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/README.md	
@@ -57,8 +57,8 @@ tags:
 <strong>输出：</strong>
 [null,false,true]
 <strong>解释：</strong>
-FindElements findElements = new FindElements([-1,null,-1]); 
-findElements.find(1); // return False 
+FindElements findElements = new FindElements([-1,null,-1]);
+findElements.find(1); // return False
 findElements.find(2); // return True </pre>
 
 <p><strong class="example">示例 2：</strong></p>
@@ -316,26 +316,24 @@ func (this *FindElements) Find(target int) bool {
  */
 
 class FindElements {
-    private s: Set<number> = new Set<number>();
+    readonly #s = new Set<number>();
 
     constructor(root: TreeNode | null) {
         root.val = 0;
-        const dfs = (root: TreeNode) => {
-            this.s.add(root.val);
-            if (root.left) {
-                root.left.val = root.val * 2 + 1;
-                dfs(root.left);
-            }
-            if (root.right) {
-                root.right.val = root.val * 2 + 2;
-                dfs(root.right);
-            }
+
+        const dfs = (node: TreeNode | null, x = 0) => {
+            if (!node) return;
+
+            this.#s.add(x);
+            dfs(node.left, x * 2 + 1);
+            dfs(node.right, x * 2 + 2);
         };
+
         dfs(root);
     }
 
     find(target: number): boolean {
-        return this.s.has(target);
+        return this.#s.has(target);
     }
 }
 
@@ -346,6 +344,53 @@ class FindElements {
  */
 ```
 
+#### JavaScript
+
+```js
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+
+const s = Symbol.for('s');
+
+/**
+ * @param {TreeNode} root
+ */
+var FindElements = function (root) {
+    root.val = 0;
+    this[s] = new Set();
+
+    const dfs = (node, x = 0) => {
+        if (!node) return;
+
+        this[s].add(x);
+        dfs(node.left, x * 2 + 1);
+        dfs(node.right, x * 2 + 2);
+    };
+
+    dfs(root);
+};
+
+/**
+ * @param {number} target
+ * @return {boolean}
+ */
+FindElements.prototype.find = function (target) {
+    return this[s].has(target);
+};
+
+/**
+ * Your FindElements object will be instantiated and called as such:
+ * var obj = new FindElements(root)
+ * var param_1 = obj.find(target)
+ */
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->
diff --git a/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/README_EN.md b/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/README_EN.md
index 8ed062fd7d793..59a00d29d1e5f 100644
--- a/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/README_EN.md	
+++ b/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/README_EN.md	
@@ -54,8 +54,8 @@ tags:
 <strong>Output</strong>
 [null,false,true]
 <strong>Explanation</strong>
-FindElements findElements = new FindElements([-1,null,-1]); 
-findElements.find(1); // return False 
+FindElements findElements = new FindElements([-1,null,-1]);
+findElements.find(1); // return False
 findElements.find(2); // return True </pre>
 
 <p><strong class="example">Example 2:</strong></p>
@@ -308,26 +308,24 @@ func (this *FindElements) Find(target int) bool {
  */
 
 class FindElements {
-    private s: Set<number> = new Set<number>();
+    readonly #s = new Set<number>();
 
     constructor(root: TreeNode | null) {
         root.val = 0;
-        const dfs = (root: TreeNode) => {
-            this.s.add(root.val);
-            if (root.left) {
-                root.left.val = root.val * 2 + 1;
-                dfs(root.left);
-            }
-            if (root.right) {
-                root.right.val = root.val * 2 + 2;
-                dfs(root.right);
-            }
+
+        const dfs = (node: TreeNode | null, x = 0) => {
+            if (!node) return;
+
+            this.#s.add(x);
+            dfs(node.left, x * 2 + 1);
+            dfs(node.right, x * 2 + 2);
         };
+
         dfs(root);
     }
 
     find(target: number): boolean {
-        return this.s.has(target);
+        return this.#s.has(target);
     }
 }
 
@@ -338,6 +336,53 @@ class FindElements {
  */
 ```
 
+#### JavaScript
+
+```js
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+
+const s = Symbol.for('s');
+
+/**
+ * @param {TreeNode} root
+ */
+var FindElements = function (root) {
+    root.val = 0;
+    this[s] = new Set();
+
+    const dfs = (node, x = 0) => {
+        if (!node) return;
+
+        this[s].add(x);
+        dfs(node.left, x * 2 + 1);
+        dfs(node.right, x * 2 + 2);
+    };
+
+    dfs(root);
+};
+
+/**
+ * @param {number} target
+ * @return {boolean}
+ */
+FindElements.prototype.find = function (target) {
+    return this[s].has(target);
+};
+
+/**
+ * Your FindElements object will be instantiated and called as such:
+ * var obj = new FindElements(root)
+ * var param_1 = obj.find(target)
+ */
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->
diff --git a/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/Solution.js b/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/Solution.js
new file mode 100644
index 0000000000000..aa02416cdf0e0
--- /dev/null
+++ b/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/Solution.js	
@@ -0,0 +1,42 @@
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+
+const s = Symbol.for('s');
+
+/**
+ * @param {TreeNode} root
+ */
+var FindElements = function (root) {
+    root.val = 0;
+    this[s] = new Set();
+
+    const dfs = (node, x = 0) => {
+        if (!node) return;
+
+        this[s].add(x);
+        dfs(node.left, x * 2 + 1);
+        dfs(node.right, x * 2 + 2);
+    };
+
+    dfs(root);
+};
+
+/**
+ * @param {number} target
+ * @return {boolean}
+ */
+FindElements.prototype.find = function (target) {
+    return this[s].has(target);
+};
+
+/**
+ * Your FindElements object will be instantiated and called as such:
+ * var obj = new FindElements(root)
+ * var param_1 = obj.find(target)
+ */
diff --git a/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/Solution.ts b/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/Solution.ts
index 66325c19fe0a0..a2fb78930175b 100644
--- a/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/Solution.ts	
+++ b/solution/1200-1299/1261.Find Elements in a Contaminated Binary Tree/Solution.ts	
@@ -13,26 +13,24 @@
  */
 
 class FindElements {
-    private s: Set<number> = new Set<number>();
+    readonly #s = new Set<number>();
 
     constructor(root: TreeNode | null) {
         root.val = 0;
-        const dfs = (root: TreeNode) => {
-            this.s.add(root.val);
-            if (root.left) {
-                root.left.val = root.val * 2 + 1;
-                dfs(root.left);
-            }
-            if (root.right) {
-                root.right.val = root.val * 2 + 2;
-                dfs(root.right);
-            }
+
+        const dfs = (node: TreeNode | null, x = 0) => {
+            if (!node) return;
+
+            this.#s.add(x);
+            dfs(node.left, x * 2 + 1);
+            dfs(node.right, x * 2 + 2);
         };
+
         dfs(root);
     }
 
     find(target: number): boolean {
-        return this.s.has(target);
+        return this.#s.has(target);
     }
 }