next.js/packages/next/src/client/app-build-id.ts

// This gets assigned as a side-effect during app initialization. Because it
// represents the build used to create the JS bundle, it should never change
// after being set, so we store it in a global variable.
//
// When performing RSC requests, if the incoming data has a different build ID,
// we perform an MPA navigation/refresh to load the updated build and ensure
// that the client and server in sync.

// Starts as an empty string. In practice, because setAppBuildId is called
// during initialization before hydration starts, this will always get
// reassigned to the actual build ID before it's ever needed by a navigation.
// If for some reasons it didn't, due to a bug or race condition, then on
// navigation the build comparision would fail and trigger an MPA navigation.
let globalBuildId: string = ''

export function setAppBuildId(buildId: string) {
  globalBuildId = buildId
}

export function getAppBuildId(): string {
  return globalBuildId
}