exports.calculate = function(expression) {
  const parts = expression.split('');

  const tracker = [];

  for (let part of parts) {
    if( part !== ''){
      tracker.push(parseFloat(part));
    } else {
      const operand2 = tracker.pop();
      const operand1 = tracker.pop();

      switch (part) {
        case '+':
        tracker.push(operand1 + operand2);
        break;
         case '-':
        tracker.push(operand1 - operand2);
        break;
         case '*':
        tracker.push(operand1 * operand2);
        break;
         case '/':
        tracker.push(operand1 / operand2);
        break;
      }

    }
  }

  return tracker.pop();
}