How to implement Swift-like enums with associated values in JavaScript?

You can use a discriminated union for this. This example uses Typescript, but the concept would be similar for Javascript just without the type safety.

interface UPCA {
    kind: "UPCA";
    numberSystem: number;
    manufacturer: number;
    item: number;
    checkDigit: number;
}

interface QRCode {
    kind: "QRCode";
    data: string;
}

interface Other {
    kind: "Other";
}

type Barcode = UPCA | QRCode | Other;

You can then switch over a value of Barcode and access the associated values after checking the discriminate.

function printBarcode(barcode: Barcode) {
  switch (barcode.kind) {
    case "UPCA":
      console.log(`UPCA: ${barcode.numberSystem}-${barcode.manufacturer}`);
      break;
    case "QRCode":
      console.log(`QRCode: ${barcode.data}`);
      break;
    case "Other":
      console.log("Other barcode");
      break;
  } 
}