check-simple-parser.js

const input = `add(if(Q1;10;0);if(equals(Q2;"high");20;if(equals(Q2;"medium");10;0));Q3)`;

const data = {
  Q1: true,
  Q2: "medium",
  Q3: 5,
};

const getParams = (str) => {
  const [OPEN, CLOSED, DELIM] = ["(", ")", ";"];
  let params = [];
  let open = 0;
  let expression = "";
  for (let i = 0; i < str.length; i++) {
    const char = str[i];
    if (char === OPEN) {
      open++;
    }

    if (char === CLOSED) {
      open--;
    }

    if (i > 0 && i < str.length - 1) {
      expression += char;
    }

    if (open === 1 && char === DELIM) {
      //we reach end
      params.push(expression.slice(0, expression.length - 1));
      expression = "";
    }

    if (open === 0) {
      params.push(expression);
    }
  }

  return params;
};

const buildTree = (str) => {
  let token = "";
  const TOKENS = ["add", "if", "equals"];

  if (str.startsWith(`"`)) {
    return {
      type: "string",
      value: str.slice(1, str.length - 1),
    };
  }

  if (str.startsWith("Q")) {
    return {
      type: "parameter",
      value: str,
    };
  }

  let number = parseInt(str, 10);
  if (!Number.isNaN(number)) {
    return {
      type: "number",
      value: number,
    };
  }

  //parse functions
  for (let i = 0; i < str.length; i++) {
    let char = str[i];
    token += char;

    if (TOKENS.indexOf(token) !== -1) {
      const params = getParams(str.slice(i + 1));

      return {
        type: token,
        values: params.map((param) => buildTree(param)),
      };
    }
  }

  return result;
};

console.log(buildTree(input));