Antlr4 visitor (JavaScript) returns array of udefined

I'm trying to make antlr4 visitor, that will return custom result from my tree. I implemented these functions:

export default class GrammarParserVisitor extends MyGrammarParserVisitor {
  // Visit a parse tree produced by MyGrammarParser#formula.
  visitFormula(ctx) {
    let result = [];

    for (let i = 0; i < ctx.getChildCount(); i++) {
      result.push(this.visit(ctx.getChild(i)));
    }

    return result;
  }

  visitFunction(ctx) {
    let result = [ctx.FUNCTION_NAME().getText()];

    const expressionList = ctx.expression();

    for (let i = 0; i < expressionList.length; i++) {
      console.log("FUNC ARGS", expressionList[i].getText());
      result.push(this.visit(expressionList[i]));
    }

    console.log(result);

    return this.result;
  }

And when i do console.log(result); in visitFunction method i get [ 'IF', [ undefined ], [ [ [Array] ] ], [ [ [Array] ] ] ]

but after visitor returns result i get [ undefined, [ [ [Array] ] ], undefined ] How to return result form visitor without undefined results?

grammar part

formula : ST expression EOF;
function
    :   FUNCTION_NAME OPEN_ROUND_BRACKET (expression (COMMA expression)*)? CLOSE_ROUND_BRACKET
    ;

and index.js with input string

const formula = "=IF($I$11=D169,1,0)";
var chars = new InputStream(formula, true);
var lexer = new MyGrammarLexer(chars);
var tokens = new CommonTokenStream(lexer);
var parser = new MyGrammarParser(tokens);
const tree = parser.formula();

const visitor = new GrammarParserVisitor();

let res = visitor.visitFormula(tree);

console.log(tree.toStringTree(parser.ruleNames));
console.log(res);

return this.result;

You've never set this.result anywhere, so this will return undefined. I assume you want to return the value of the local result variable (the same one you're printing on the line before), not some field, so cut the this..