Stubbing fs using sinonjs in typescript

fs-stubbing.test.ts

import testModule from '../src/test';
import { expect } from 'chai';
import sinon from 'sinon';
import fs from 'fs';

describe('testModule', () => {
  let readFileSyncStub: sinon.SinonStub;
  let sandbox;

  beforeEach(() => {
    sandbox = sinon.sandbox.create();
    readFileSyncStub = sandbox.stub(fs, 'readFileSync')
  });

  afterEach(() => {
    sandbox.restore();
  });

  it('should return blah', () => {
    readFileSyncStub.returns('blah blah');
    const result = testModule('./some/file.txt');
    expect(result).to.equal('blah blah');
  });

});

fs-stubbing.ts

import fs from "fs";

export default function(pathToFile: string) {
  return fs.readFileSync(pathToFile);
}