Balancing Brackets Algorithm: Stack Data Structure
The code looks reasonably clean, IMO. I assume that it is purposely ES5?

I don't know about Stack as it's not a ES5 standard API (that I know of).

You are not returning any value.  There is no need for ? true : false — just do return stack.length() === 0;.

Some code that you can consider, in ES5.



var completes = ['()', '[]', '{}'];
var open = completes.map(function(complete) {
  return complete.charAt(0);
});
var close = completes.map(function(complete) {
  return complete.charAt(1);
});

function isBalanced(arr) {
  var stack = [];
  arr.forEach(function(curr) {
    if (open.indexOf(curr) !== -1) {
      stack.push(curr);
    }
    if (close.indexOf(curr) !== -1) {
      var last = stack.pop();
      if (completes.indexOf(last + curr) === -1) {
        stack.push(last);
      }
    }
  });
  return stack.length === 0;
}

console.log(isBalanced(['{', '(', ')', ']']));
console.log(isBalanced(['[', '(', ')', ']']));