hasBalancedBrackets.js

const bracketList = '[]{}()'
const bracketNumber = bracketList.length / 2

const hasBalancedBrackets = str => {
  const stack = []

  let bracketOpenLastIndex = null
  let bracketOpenIndex = null
  let bracketCloseIndex = null

  for (let bi = 0; bi < bracketNumber; bi++) {
    bracketOpenIndex = bi * 2
    bracketCloseIndex = bi * 2 + 1
    for (let si = 0; si < str.length; si++) {
      if (str[si] == bracketList[bracketOpenIndex]) {
        stack.push(bracketOpenIndex)
        bracketOpenLastIndex = bracketOpenIndex
      }
      if (str[si] == bracketList[bracketCloseIndex]) {
        if (bracketOpenLastIndex !== bracketOpenIndex || !stack.length) {
          return false
        }
        stack.pop()
        bracketOpenLastIndex = stack[stack.length - 1]
      }
    }
  }

  return !stack.length;
}

const testCases = {
  positive: ['', '_', '[_]', '{_}', '[_{_}_]', '[_(_{_}_)_]'],
  negative: ['[', '[{]', '[{[{]}]']
}

testCases.positive.forEach(testCase => console.assert(hasBalancedBrackets(testCase)))
testCases.negative.forEach(testCase => !console.assert(!hasBalancedBrackets(testCase)))