fix: use const to declare variables in js

codes/javascript/chapter_stack_and_queue/stack.js

@@ -16,13 +16,13 @@ stack.push(5);
 stack.push(4);
 
 /* 访问栈顶元素 */
-peek = stack[stack.length - 1];
+const peek = stack[stack.length - 1];
 
 /* 元素出栈 */
-pop = stack.pop();
+const pop = stack.pop();
 
 /* 获取栈的长度 */
-size = stack.length;
+const size = stack.length;
 
 /* 判断是否为空 */
-is_empty = stack.length === 0;
+const is_empty = stack.length === 0;

docs/chapter_stack_and_queue/stack.md

@@ -155,16 +155,16 @@ comments: true
     stack.push(4);
 
     /* 访问栈顶元素 */
-    peek = stack[stack.length-1];
+    const peek = stack[stack.length-1];
 
     /* 元素出栈 */
-    pop = stack.pop();
+    const pop = stack.pop();
 
     /* 获取栈的长度 */
-    size = stack.length;
+    const size = stack.length;
 
     /* 判断是否为空 */
-    is_empty = stack.length === 0;
+    const is_empty = stack.length === 0;
     ```
 
 === "TypeScript"