Jan's Notation

function solve(arr) {
  let operands = [];
  let operators = [];

  for (const ele of arr) {
    if (checkNumberOrNot(ele)) {
      operands.push(ele);
    } else {
      if (operands.length > 1) {
        const operand2 = operands.pop();
        const operand1 = operands.pop();
        let newOperand = calculate(operand1, operand2, ele);
        operands.push(newOperand);
      } else {
        operators.push(ele);
      }
    }
  }

  let breakCondition = (operands.length !== operators.length)? "Error: too many operands!" : "Error: not enough operands!";
  return (operands.length === 1 && operators.length == 0)? operands[0]: breakCondition;

  function calculate(a, b, op) {
    if (op === '+') {
      return a + b;
    }else if (op === '-') {
      return a - b;
    } else if (op === '*') {
      return a * b;
    } else if (op === '/') {
      return a / b;
    }
  }

  function checkNumberOrNot(x) {
    if (typeof x === 'number') {
      return true;
    } else {
      return false;
    }
  }
}