Interview Question Answers from https://performancejs.com/post/hde6d32/Big-List-of-Frontend-JavaScript-Interview-Questions-(by-a-Frontend-Engineer)

isBalanced2.js

isBalanced = (() => {
  const braces = {
    '{': [ 0,  1 ],
    '}': [ 0, -1 ],
    '[': [ 1,  1 ],
    ']': [ 1, -1 ],
    '(': [ 2,  1 ],
    ')': [ 2, -1 ]
  }
  return function isBalanced (s) {
    const counts = [ 0, 0, 0 ]
    const order = []
    for (let c of s) {
      if (!braces.hasOwnProperty(c)) continue;
      const [ type, value ] = braces[c]
      counts[type] += value
      if (value < 0 && order.pop() !== type) {
        return false
      } else if (value > 0) {
        order.push(type)
      }
    }
    return !order.length
  }
})()

/* Test */
isBalanced('(foo { bar (baz) [boo] })') // true
isBalanced('foo { bar { baz }')         // false
isBalanced('foo { (bar [baz] } )')      // false