Checking if parentheses are balanced
I linked to a very related question in a comment, and this will repeat some points from my answer to that other question. Still, there are some other things here.


Don't do stack.push(c). Do stack.push(symbolPosition + 1) instead. I.e. push the expected position, not the current character. When you just push the character, you have to call indexOf again later to get that character's symbol position when you pop it off of the stack. By just pushing the position to the stack to begin with, you avoid that. And by adding 1 when pushing it, you get a cleaner conditional, since you can just do stack.pop() !== symbolPosition
If you do the above, you don't need stack.length &lt; 1 before the stack.pop() call either. Nothing will break if the stack's empty; pop() will just return undefined which won't match any symbol position, and thus trigger the return false.
I'd skip the isOpen variable. Sure, it might help explain what that modulo operation means, but you only need the result once. I'd rather put the explanation in a comment, and skip the single-use variable.
Similarly, I'd skip the c variable, which - if you follow the first point above - is also only used once (and whose single-letter name doesn't help explain much), and just do indexOf(str[i]).
I'd change the last line to return stack.length === 0 just to be explicit. Yes the result is the same, but the intent is clearer in my opinion. "Is the stack empty?" versus "Is the stack's length less than 1?". You could also do return !stack.length but that's unnecessarily cryptic.


In the end you get something like:

function isBalanced(str) {
    var stack = [];
    const allowedSymbols = '()[]&lt;&gt;';

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

        if(!~symbolPosition) {
            continue;
        }

        if(symbolPosition % 2 === 0) {
            stack.push(symbolPosition + 1);
        } else {
            if(stack.pop() !== symbolPosition) {
                return false;
            }
        }
    }

    return stack.length === 0;
}


As others have pointed out, you can do some tweaking with regard to const vs let vs var, but I'll leave that to you.