This script checks if parentheses are balanced. I wonder if there is something that can be improved here including ES6 features.

function isBalanced(str) {
    var stack = [];
    const allowedSymbols = '()[]<>';

    for(let i = 0, l = str.length; i < l; i++) {
        var c = str[i];
        var isOpen;
        var symbolPosition = allowedSymbols.indexOf(c);

        if(!~symbolPosition) {
            continue;
        }

        isOpen = symbolPosition % 2 ? false : true;

        if(isOpen) {
            stack.push(c);
        } else {
            if(stack.length < 1 || allowedSymbols.indexOf(stack.pop()) !== symbolPosition - 1) {
                return false;
            }
        }
    }

    return stack.length < 1;
}

console.log('()', isBalanced('()')); //true
console.log(')(', isBalanced(')(')); //false