这是UT解决方案：

index.ts：

import fs from 'fs';

export const viewText = () => {
  fs.readFile('poem.txt','utf8',(err,data) => {
    if (err) throw err;
    console.log(data);
  });
};

index.spec.ts：

import { viewText } from './';
import fs from 'fs';

const mockPoem =
  'Some say the world will end in fire,Some say in ice. From what I’ve tasted of desire I hold with those who favor fire ... And would suffice.';

describe('viewText',() => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  test('prints poem to console',done => {
    const logSpy = jest.spyOn(console,'log');
    let readFileCallback;
    // @ts-ignore
    jest.spyOn(fs,'readFile').mockImplementation((path,options,callback) => {
      readFileCallback = callback;
    });

    viewText();
    readFileCallback(null,mockPoem);
    expect(logSpy).toBeCalledWith(mockPoem);
    expect(fs.readFile).toBeCalledWith('poem.txt',readFileCallback);
    done();
  });

  test('should throw error when read file failed',done => {
    let readFileCallback;
    // @ts-ignore
    jest.spyOn(fs,callback) => {
      readFileCallback = callback;
    });

    viewText();
    const mError = new Error('read file failed');
    expect(() => readFileCallback(mError,null)).toThrowError(mError);
    expect(fs.readFile).toBeCalledWith('poem.txt',readFileCallback);
    done();
  });
});

覆盖率100％的单元测试结果：

 PASS  src/stackoverflow/58810079/index.spec.ts (11.118s)
  viewText
    ✓ prints poem to console (23ms)
    ✓ should throw error when read file failed (3ms)

  console.log node_modules/jest-mock/build/index.js:860
    Some say the world will end in fire,Some say in ice. From what I’ve tasted of desire I hold with those who favor fire ... And would suffice.

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.ts |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed,1 total
Tests:       2 passed,2 total
Snapshots:   0 total
Time:        13.129s

源代码：https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/58810079