A little parser 2. evaluating

little-parser-ii.js

/* evaluate a tree of results. */
function evalResult(result) {
  if (result.type === exps.NumOrGroup) {
    /* ex: (123 + 456) */
    return evalResult(result.parts[0]);
  } else if (result.type === exps.NumLiteral) {
    /* from the string "123" into a number. */
    return parseFloat(result.parts[0]);
  } else {
    /* reduce/evaluate the expression */
    let [operator, ...exprs] = result.parts;
    let val = evalResult(exprs[0]);
    for (let i = 1; i < exprs.length; i++) {
      if (operator === "+") {
        val += evalResult(exprs[i]);
      } else if (operator === "*") {
        val *= evalResult(exprs[i]);
      } else if (operator === "^") {
        val = Math.pow(val, evalResult(exprs[i]));
      } else {
        throw new Error("Unknown operator.");
      }
    }

    return val;
  }
}

/* how to run and evaluate an expression */
function demo() {
  let got = evalResult(parseTotal(verySimpleLexer("2 + 3 * 4")));
  console.log(`2 + 3 * 4 = ${got}`);
}