is-balanced.js

console.clear()

const OPEN_ROUND = '('
const CLOSE_ROUND = ')'

const OPEN_CURLY = '{'
const CLOSE_CURLY = '}'

const OPEN_SQUARE = '['
const CLOSE_SQUARE = ']'

const closes = (open, close) => {
  if (open === OPEN_ROUND && close === CLOSE_ROUND) {
    return true
  }
  
  if (open === OPEN_CURLY && close === CLOSE_CURLY) {
    return true
  }
  
  if (open === OPEN_SQUARE && close === CLOSE_SQUARE) {
    return true
  }
  
  return false
}

const isBalanced = (string) => {
  const stack = []
  
  for (let i = 0; i < string.length; i++) {
    const character = string[i]
    switch (character) {
      case OPEN_ROUND:
      case OPEN_CURLY:
      case OPEN_SQUARE:
        stack.push(character)
        break
      case CLOSE_ROUND:
      case CLOSE_CURLY:
      case CLOSE_SQUARE:
        const popped = stack.pop()
        if (stack.length === 0 && !closes(popped, character)) {
          return false
        }
        break
      default:
        throw new Error('Unexpected character: ' + character)
    }
  }
  
  return true
}

['[[]]', '()', ')()', '[[(])]'].forEach(string => { console.log(isBalanced(string)) })