Using v20.0.0-beta3 with nodejs

.babelrc

{
  "presets": ["@babel/preset-env"]
}

index.js

import { Api, JsonRpc } from 'eosjs';
import JsSignatureProvider from 'eosjs/dist/eosjs-jssig';  // development only
import fetch from 'node-fetch';                            // node only; not needed in browsers

const privateKeys = ["privateKeyHere"];

const signatureProvider = new JsSignatureProvider(privateKeys);

console.log(signatureProvider)

package.json

{
  "name": "eosjs-test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "start": "nodemon --exec babel-node index.js",
  },
  "author": "szendvics",
  "license": "ISC",
  "dependencies": {
    "eosjs": "20.0.0-beta3"
  },
  "devDependencies": {
    "@babel/cli": "^7.0.0-rc.1",
    "@babel/core": "^7.0.0-rc.1",
    "@babel/node": "^7.0.0-rc.1",
    "@babel/preset-env": "^7.0.0-rc.1",
    "nodemon": "^1.18.3"
  }
}