basic expresion execution

expresion_parser.js

const token_precedence = {
  "-": 1,
  "+": 1,
  "*": 2,
  "/": 2,
  "^": 3,
  "n": 4,
  "(": 5,
  ")": 5,
};

function parse_exp_to_rev_pol(exp) {
  /// tokenise
  const tokens = [];
  for (let i = 0; i < exp.length; i++) {
    if (exp[i] == "-" && (i == 0 || exp[i-1] < "0" || exp[i-1] > "9" || exp[i-1] == "(")) {
      tokens.push("n");
    }else if (exp[i] >= "0" && exp[i] <= "9") {
      let number = "";
      while (exp[i] >= "0" && exp[i] <= "9") {
        number += exp[i];
        i++;
      }
      i--;
      tokens.push(Number(number));
    } else {
      tokens.push(exp[i]);
    }
  }

  console.log(tokens);

  const output = [];
  const ops = [];
  for (let i = 0; i < tokens.length; i++) {
    if (typeof tokens[i] === "number") {
      output.push(tokens[i]);
    } else if (tokens[i] == "("){
      ops.push(tokens[i]);
    } else if (tokens[i] == ")"){
      while (true) {
        const top = ops.pop();
        if (typeof top === "undefined") {
          throw "Mismatched brackets";
        }
        if (top == "(") { break; }
        output.push(top);
      }
    } else {
      while (ops.length > 0 && ops.at(-1) != "(" && token_precedence[ops.at(-1)] > token_precedence[tokens[i]]) {
        output.push(ops.pop());
      }
      ops.push(tokens[i]);
    }
  }
  while (ops.length > 0){
    const val = ops.pop();
    if (val == "(") {
      throw "Mismatched brackets";
    }
    output.push(val);
  }

  return output;
}
function compute_reverse_polish(tokens){
  const val = tokens.pop();

  if (typeof val === "number") {
    return val;
  }

  if (val == "n") {
    return -compute_reverse_polish(tokens);
  } else if (val == "^") {
    const rhs = compute_reverse_polish(tokens);
    return compute_reverse_polish(tokens) ** rhs;
  } else if (val == "/") {
    const rhs = compute_reverse_polish(tokens);
    return compute_reverse_polish(tokens) / rhs;
  } else if (val == "*") {
    const rhs = compute_reverse_polish(tokens);
    return compute_reverse_polish(tokens) * rhs;
  } else if (val == "+") {
    const rhs = compute_reverse_polish(tokens);
    return compute_reverse_polish(tokens) + rhs;
  } else if (val == "-") {
    const rhs = compute_reverse_polish(tokens);
    return compute_reverse_polish(tokens) - rhs;
  } else {
    console.log(val)
    throw "ERROR????"
  }
}

function compute_exp(exp) {
  const tokens = parse_exp_to_rev_pol(exp);
  const result = compute_reverse_polish(tokens);
  if (tokens.length > 0) {
    throw "invalid expression";
  }
  return result
}