Valid Parenthesis - https://www.codewars.com/kata/valid-parentheses/javascript

validParemthesesFunctional.js

// solution with functional programming

function validParentheses(cad) {
  
  if (cad.length === 0) {
    return true;
  }
  
  if (cad[0] === ')' || cad[cad.length - 1] === '(') {
    return false;
  } else {
    if (cad[1] === ')') {
      return validParentheses(cad.slice(2));
    } else {
      var index = cad.indexOf(')');
      return validParentheses(cad.slice(0, index - 1) + cad.slice(index + 1));
    }
  }
}

validParentheses.js

// solution initial

function validParentheses(cad) {
  let arr = cad.split('');

  while (arr.length !== 0) {
    let band = true;
    for (let i = 0; i < arr.length - 1; i++) {
      if (arr[i] === '(' && arr[i + 1] === ')') {
        arr.splice(i, 2);
        band = false;
        break;
      }
    }
    if (band) break;
  }

  return arr.length === 0;
}