Finding the longest valid parentheses JavaScript

Given a string containing just the characters '(' and ')', we find the length of the longest valid (well-formed) parentheses substring.

A set of parentheses qualifies to be a well-formed parentheses, if and only if, for each opening parentheses, it contains a closing parentheses.

For example −

'(())()' is a well-formed parentheses
'())' is not a well-formed parentheses
'()()()' is a well-formed parentheses

Example

const str = '(())()(((';
   const longestValidParentheses = (str = '') => {
      var ts = str.split('');
      var stack = [], max = 0;
      ts.forEach((el, ind) => {
         if (el == '(') {
            stack.push(ind);
         }
      else {
         if (stack.length === 0 || ts[stack[stack.length - 1]] == ')'){
            stack.push(ind);
         }
         else {
            stack.pop();
         };
      }
   });
   stack.push(ts.length);
   stack.splice(0, 0, -1);
   for (let ind = 0;
   ind
Output
And the output in the console will be −
6