EvaluateMathematicalExpression

var run = function (str) {
  return expression(str);
};

var peek = function (str) {
  return str[0] || '';
};

var get = function (str) {
  return str.shift();
};



var factor = function (str) {
    
  function makeNumber(str) {
  var result = '';
  while(/[0-9\.]/.test(peek(str))) {
    result += get(str);
  }
  return Number(result);
}
    
  if(peek(str) === "-") {
    // rule - factor
    get(str);
    return -1 * factor(str);
  } else if(!isNaN(peek(str))) {
    return makeNumber(str);
  } else if(peek(str) === "(") {
    get(str);  //consume opening parenthesis
    var number = expression(str);
    get(str); // consume closing parenthesis
    return number;
  }
};

var term = function (str) {
  var result = factor(str);
  while (peek(str) == '*' || peek(str) == '/') {
    if (get(str) == '*') {
      result *= factor(str);
    } else {
      result /= factor(str);
    }
  }
  return result;
};

var expression = function (str) {
  var result = term(str);
  while (peek(str) == '+' || peek(str) == '-') {
    if (get(str) == '+') {
      result += term(str);
    } else {
      result -= term(str);
    }
  }
  return result;
};

var calc = function (expression) {
  var expressionToParse = expression.replace(/\s+/g, '').split('');
  return run(expressionToParse);
}

console.log(calc("3+4+5"))