Method withImplementation

packages/jest-mock/src/index.ts:812–835

(
        this: ModuleMocker,
        fn: T,
        callback: (() => void) | (() => Promise<unknown>),
      )

        callback: () => Promise<unknown>,
      ): Promise<void>;
      function withImplementation(
        this: ModuleMocker,
        fn: T,
        callback: (() => void) | (() => Promise<unknown>),
      ): void | Promise<void> {
        // Remember previous mock implementation, then set new one
        const mockConfig = this._ensureMockConfig(f);
        const previousImplementation = mockConfig.mockImpl;
        const previousSpecificImplementations = mockConfig.specificMockImpls;
        mockConfig.mockImpl = fn;
        mockConfig.specificMockImpls = [];

        const returnedValue = callback();

        if (isPromise(returnedValue)) {
          return returnedValue.then(() => {
            mockConfig.mockImpl = previousImplementation;
            mockConfig.specificMockImpls = previousSpecificImplementations;
          });
        } else {
          mockConfig.mockImpl = previousImplementation;
          mockConfig.specificMockImpls = previousSpecificImplementations;
        }
      }

      f.mockImplementation = (fn: T) => {
        // next function call will use mock implementation return value