BracketsIsCorrected

let brackets = {
  "[": "]",
  "{": "}",
  "(": ")",
  "<": ">"
}

function correctBrackets (str) {
  const vals = Object.keys(brackets).map(key => brackets[key]);
  const keys = Object.keys(brackets);
  
  if (vals.indexOf(str[0]) + 1) {
    return null;
  }
  
  let mass = str.split('');
  let stack = [];
  
  for (let i = 0; i < mass.length; i++) {
    const l = mass[i];
    
    if (keys.indexOf(l) + 1) {
      stack.push(l);
    } else {
      if (stack.length) {
        mass[i] = brackets[stack.shift()];
      } else {
        return null;
      }
    }
  }
  
  if (stack.length) {
    return null;
  }
  
  return mass.join('');
}

function isValidBrackets(str) {

  if (str.length <= 1)
    return false

  let matchingOpeningBracket, ch
  let stack = []

  let openingBrackets = ['[', '{', '(']
  let closingBrackets = [']', '}', ')']

  for (let i = 0; i < str.length; i++) {
    ch = str[i]

    if (closingBrackets.indexOf(ch) > -1) {
      matchingOpeningBracket = openingBrackets[closingBrackets.indexOf(ch)]
      if (stack.length == 0 || (stack.pop() != matchingOpeningBracket)) {
        return false
      }
    } else {
      stack.push(ch)
    }
  }

  return (stack.length == 0)
};