import {
  MyGrammarLexer,
  MyGrammarParser,
  MyGrammarListener,
  MyGrammarVisitor
} from './MyGrammar.g4';

module.exports = {
  module: {
    rules: [
      { test: /\.g4/, loader: 'antlr4-webpack-loader' }
    ]
  }
};

import { InputStream, CommonTokenStream } from 'antlr4/index';
import { MyGrammarLexer, MyGrammarParser, MyGrammarVisitor } from './MyGrammar.g4';

class MyVisitor extends MyGrammarVisitor {

  visitMyStartRule(ctx) { ... }

  ...
}

const input = ...; // Load string content
const lexer = new MyGrammarLexer(new InputStream(input));
const parser = new MyGrammarParser(new CommonTokenStream(lexer));
const result = new MyVisitor().visit(parser.myStartRule());