vue3-core/packages/runtime-core/src/helpers/useId.ts

import {
  type ComponentInternalInstance,
  getCurrentInstance,
} from '../component'
import { warn } from '../warning'

export function useId(): string {
  const i = getCurrentInstance()
  if (i) {
    return (i.appContext.config.idPrefix || 'v') + '-' + i.ids[0] + i.ids[1]++
  } else if (__DEV__) {
    warn(
      `useId() is called when there is no active component ` +
        `instance to be associated with.`,
    )
  }
  return ''
}

/**
 * There are 3 types of async boundaries:
 * - async components
 * - components with async setup()
 * - components with serverPrefetch
 */
export function markAsyncBoundary(instance: ComponentInternalInstance): void {
  instance.ids = [instance.ids[0] + instance.ids[2]++ + '-', 0, 0]
}