Correct Brackets Problem (Catalan numbers task group)

<script>
function checkBrackets(str){
    var openedCounter = 0;
    var closedCounter =  0;
    var isCorrect = true;
    for (var i = 0; i <= str.length; i++){
        if (str[i] === '('){
          openedCounter++;
        }
        if (str[i] === ')'){
          closedCounter++;
        }
        if (openedCounter < closedCounter){
            return false;
        }
    }
    if (openedCounter !== closedCounter){
        return false;
    }

    return true;
}

var testStr = '((a+b)/5-d)';
console.log(testStr);
console.log('Are the brackets in the expression correct? ' + checkBrackets(testStr));

var testStr2 = ')(a+b))';
console.log(testStr2);
console.log('Are the brackets in the expression correct? ' + checkBrackets(testStr2));

var testStr3 = '()(a+b))(';
console.log(testStr3);
console.log('Are the brackets in the expression correct? ' + checkBrackets(testStr3));
</script>