vue3-core/packages/runtime-core/src/apiInject.ts

import { currentInstance } from './component'

export interface InjectionKey<T> extends Symbol {}

export function provide<T>(key: InjectionKey<T> | string, value: T) {
  if (!currentInstance) {
    // TODO warn
  } else {
    let provides = currentInstance.provides
    // by default an instance inherits its parent's provides object
    // but when it needs to provide values of its own, it creates its
    // own provides object using parent provides object as prototype.
    // this way in `inject` we can simply look up injections from direct
    // parent and let the prototype chain do the work.
    const parentProvides =
      currentInstance.parent && currentInstance.parent.provides
    if (parentProvides === provides) {
      provides = currentInstance.provides = Object.create(parentProvides)
    }
    provides[key as any] = value
  }
}

export function inject<T>(key: InjectionKey<T> | string): T | undefined
export function inject<T>(key: InjectionKey<T> | string, defaultValue: T): T
export function inject(key: InjectionKey<any> | string, defaultValue?: any) {
  if (!currentInstance) {
    // TODO warn
  } else {
    // TODO should also check for app-level provides
    const provides = currentInstance.parent && currentInstance.provides
    return provides && key in provides
      ? (provides[key as any] as any)
      : defaultValue
  }
}