写一个方法判断大括号{}是否闭合

var isClosed = function(str, tag = ['{', '}']) {  const stack = [];  for (let i in str) {    const val = str[i];    if (tag[0] === val) {      stack.push(val);    } else if (tag[1] === val) {      if (stack.length === 0) {        return false;      } else {        stack.pop();      }    }  }  return stack.length === 0;};

采取的是遍历字符串str，判断左括号{则入栈，匹配右括号}出栈
不过需要确认

isClosed('')

isCLosed('{}{}')