validate matching characters in string

validate-matching-chars.ts

const pairs = {
  '{': '}',
  '(': ')',
  '[': ']',
  '<': '>',
}

function isValid(input: string) {
  const chars = input.split('');
  let stack: string[] = [], last: string;
  do {
    last = chars.shift();
    if (stack.length > 0 && pairs[stack[stack.length-1]] === last) {
      stack.pop();
    } else {
      stack.push(last);
    }
  } while (chars.length);

  return stack.join('').length === 0;
}

// tests
isValid('{[({}{}())]}'); // true
isValid('{[({}{}())][]]}'); // false
isValid('{[({)}{}())][]]}'); // false