isBalancedBracketSequence.js

var closingMap = {
	'{': '}',
	'[': ']',
	'(': ')'
};

function isBalancedBracketSequence(expression) {
	var length = expression.length;
	if (length <= 1 || length % 2 !== 0) {
		return false;
	}
	var stack = []; //represents the bracket level. Should be zero at the end of a balanced bracket sequence
	var expression = expression.split('');
	for (var i = 0; i < expression.length; i++) { //for each bracket, test if it's opening or closing
		var bracket = expression[i];
		if (closingMap[bracket]) { //if it's opening...
			stack.push(closingMap[bracket]); //push it into the stack. We owe one closing bracket
		} else { //if it's a closing bracket, we have to pop it from the stack
			if (stack.length === 0 || stack[stack.length - 1] !== bracket)
				return false; // BUT if the stack is zeroed or is not the same type, it's unmatched
			stack.pop();
		}
	}
	if (stack.length) //if there's something left in the stack, it's unmatched
		return false;
	else
		return true;
}