How to unit test an AWS SDK method called in a separate method? - node.js

Question

I am currently writing a piece of code which simply uploads a file to an S3 bucket. This is a serverless NodeJS project written in Typescript. For testing, I am using Mocha, Chai, and attempting to use SinonJS.
I have a class with a method which uploads a buffer as a file to S3:
import { S3 } from "aws-sdk"
import { S3UploadError } from "../errors/S3UploadError"
/**
* This class provides a means of interacting with S3
*
* #export
* #class AWSS3Manager
*/
export class AWSS3Manager {
/**
* Instance of S3
*
* #private
* #type {S3}
* #memberof AWSS3Manager
*/
private s3: S3
/**
* Creates an instance of AWSS3Manager.
*
* #memberof AWSS3Manager
*/
public constructor() {
this.s3 = new S3()
}
/**
* Upload a file to bucket on S3 by using a buffer
*
* #param {Buffer} body
* #param {string} bucketName
* #param {string} fileName
* #returns {Promise<PromiseResult<S3.PutObjectOutput, S3UploadError>>}
* #memberof AWSS3Manager
*/
public async upload(body: Buffer, bucketName: string, fileName: string) {
const params = {
Body: body,
Bucket: bucketName,
Key: fileName,
}
try {
return await this.s3.putObject(params).promise()
} catch (err) {
console.log(err)
throw new S3UploadError()
}
}
}
I'm not even sure if this is the right thing to be doing, but I want to test the upload method of the above class. And in my mind, to do that I need to mock the response from S3.putObject in order to ensure that the external library doesn't cause interference. However, no matter what I have tried I can't get it to work.
I have tried stubbing the S3 put object method, but to no avail:
chai.should()
const assert = chai.assert
const expect = chai.expect
const s3manager = new AWSS3Manager()
let sandbox: sinon.SinonSandbox
let spy
describe("AWSS3Manager behaves as expected", () => {
beforeEach(() => {
sandbox = sinon.createSandbox()
spy = sinon.spy()
})
afterEach(() => {
// Restore the default sandbox here
sandbox.restore()
})
it("Uploads a file correctly to S3", async () => {
const putObjectStub = sinon.stub(S3.prototype, "putObject")
putObjectStub.yields("ERROR", 'data')
//const uploadStub = sandbox.stub(s3manager, "upload").resolves("Yup")
//sandbox.stub(AWS, "S3").resolves('HEYY')
const test = await s3manager.upload(new Buffer("ddds"), "TestBucket", "Test")
assert(putObjectStub.called)
})
})
I have tried stubbing the S3 method, and also stubbing the upload method its self and seeing how many calls there was to putObject but I can't get nothing to work.
Can anyone help?

Answer 1

I ran into a similar situation attempting to stub out the putObject call on an S3 instance that I was then returning chained with .promise(). See below:
export class UploadService {
s3: S3
constructor() {
this.s3 = new S3({ region: 'us-west-2' })
}
upload({ data, path }: { data: string, path: string }) {
const uploadParams = {
Body: data,
Bucket: bucketName, // defined elsewhere
Key: path
}
return this.s3.putObject(uploadParams).promise()
}
}
I dug into the aws-sdk definitions and came up with this test setup--I don't love it because it's kinda exposing the guts of the sdk that you shouldn't have to think about but it passes the typescript compiler and gets the job done 🤷‍♂️
import { expect } from 'chai'
import * as sinon from 'sinon'
import { UploadService } from '../../src/services/upload_service'
import { Request, Service } from 'aws-sdk'
describe('UploadService', () => {
const uploadService = new UploadService()
let uploadStub: sinon.SinonStub
describe('#upload', () => {
beforeEach(() => {
uploadStub = sinon
.stub(uploadService.s3, 'putObject')
.returns(new Request(new Service(), 'put'))
})
it('uploads the data to the specified path in S3', async () => {
const data = '{"cool": "data"}'
const path = 'my/cool/file/path'
await uploadService.upload({ data, path })
expect(uploadStub).to.have.been.calledOnceWith({ data, path })
})
})
})
Hope that helps!