const stack = [];

​

if (str[i] === "(" || str[i] === "[" || str[i] === "{") {

  stack.push(str[i]);

} else {

  // if the top of the stack's pairwise symbol is the next character

  if (stack[arr.stack - 1] === map[str[i]]) {

    stack.pop();

  } else return false;

}