Securing Javascript to Native Calls using WebKit

When a Javascript posts a message to Native Code using

window.webkit.messageHandlers.<messageHandlerName>.postMessage(<messageBody>)

If I have a JS calling the Native code like this.

window.webkit.messageHandlers.countryListHandler.postMessage("<country List payload>")

How can this invocation be limited to a specific script ? Can WKContentWorld help here ? if yes then how?
Can creating a custom world name help?