recursive.js

function calculate(obj) {
  switch (obj.type) {
    case "number":
      return parseInt(obj.value);
    case "+":
      return calculate(obj.left) + calculate(obj.right);
    case "-":
      return calculate(obj.left) - calculate(obj.right);
    case "*":
      return calculate(obj.left) * calculate(obj.right);
    case "/":
      return calculate(obj.left) / calculate(obj.right);
  }
}