Basic Calculator.js

/**
Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

You may assume that the given expression is always valid.

Some examples:
"1 + 1" = 2
" 2-1 + 2 " = 3
"(1+(4+5+2)-3)+(6+8)" = 23
Note: Do not use the eval built-in library function.
*/
/**
 * @param {string} s
 * @return {number}
 */
var calculate = function(s) {
    var stack = [],
        len = s.length,
        sum = 0,
        num,
        ch,
        j,
        i;
    
    stack.push(1);
    stack.push(1);
    
    for (i = 0; i < len; i++) {
        ch = s.charAt(i);
        
        if (!isNaN(parseInt(ch))) {
            num = parseInt(ch);
            
            for (j = i + 1; j < len && !isNaN(parseInt(s.charAt(j))); j++) {
                num = num * 10 + parseInt(s.charAt(j));
            }
            
            sum += stack.pop() * num;
            
            i = j - 1;
        } else if (ch === '+' || ch === '(') {
            stack.push(stack[stack.length - 1]);
        } else if (ch === '-') {
            stack.push(stack[stack.length - 1] * (-1));
        } else if (ch === ')') {
            stack.pop();
        }
    }
    
    return sum;
};