I assumed this block would do some type check for me.

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// types

// snippet including only public info from LSPAY Payment API
type PaymentStepEvent = PaymentStepSetup | PaymentStepData

interface PaymentStepSetup {
  step: PAYMENT_STEPS.SETUP
  // ... some omitted fields
}

interface PaymentStepData {
  step: PAYMENT_STEPS.DATA
  // ... some omitted fields
}

enum PAYMENT_STEPS {
  DATA = 'DATA',
  SETUP = 'SETUP',
}
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// event handling logic
let eventData: PaymentStepEvent
try {
  eventData = JSON.parse(event.data)
} catch (e) {
  // some error handling
}

The eventData: PaymentStepEvent looked like assigning the result of JSON.parse(event.data) to eventData would pass some type checking, but the sad truth is not.

Execute this block in playground:

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let data = '{"dog": "DATA"}' // or "{\"step\": \"DATA\"}"
let eventData: PaymentStepEvent
try {
  eventData = JSON.parse(data)
  console.log(eventData) // 👉 {"dog": "DATA"}
  console.log(
    'is recognized gateway message event: ' +
      !!isRecognizedGatewayMessageEventData(eventData)
  ) // 👉 "is recognized gateway message event: false"
} catch (e) {
  console.log('error parsing!')
}

function isPaymentStepEvent(eventData: PaymentStepEvent): boolean {
  return eventData.step && Object.values(PAYMENT_STEPS).includes(eventData.step)
}

The isPaymentStepEvent serves as a type guard, which has to be implemented by user. TypeScript adds no type checking at run time at all.