Mock AWS S3 in JavaScript using Mocha and Sinon

test.js

const SomeService = require('some/service')
const AWS = require('aws-sdk')

describe('someService', async function() {
  let someService

  beforeEach(async function() {
    someService = new SomeService()
  })

  describe('upload()', async function() {
    it('uploads an file', async function() {
      const s3Url = 'https://dummy-bucket.amazonaws.com/somefile.txt'
      this.sinon.stub(AWS, 'S3').callsFake(() => {
        const upload = function() {
          const promise = async function() {
            return { Location: s3Url }
          }
          return { promise }
        }
        return { upload }
      })

      const url = await someService.upload('test/fixtures/sample-image.jpg')
      expect(url).to.be.eql(s3Url)
    })
  })
})