Use Sinon.fakeServer with promises and mocha - node.js

Question

My problem is the following: I want to test a method that uploads a buch of data into an AWS S3 bucket. The problem is: I don't want to really upload data every time I am testing and I don't want to care about credentials sitting in the env. So I want to setup Sinon's fake-server module to simulate the upload and return the same results then S3 would. Sadly, it seems to be difficult to find a working example with code using async/await.
My test looks like this:
import {skip, test, suite} from "mocha-typescript";
import Chai from "chai";
import {S3Uploader} from "./s3-uploader.class";
import Sinon from "sinon";
#suite
class S3UploaderTest {
public server : Sinon.SinonFakeServer | undefined;
before() {
this.server = Sinon.fakeServer.create();
}
after() {
if (this.server != null) this.server.restore();
}
#test
async "should upload a file to s3 correctly"(){
let spy = Sinon.spy();
const uploader : S3Uploader = new S3Uploader();
const upload = await uploader.send("HalloWelt").toBucket("onetimeupload.test").toFolder("test/hw.txt").upload();
Chai.expect(upload).to.be.a("object");
}
}
Inside of the uploader.upload() method, I resolved a promise out of a callback. So how can I simulate the uploading-process?
Edit: Here is the code of the s3-uploader:
import AWS from "aws-sdk";
export class S3Uploader {
private s3 = new AWS.S3({ accessKeyId : process.env.ACCESS_KEY_ID, secretAccessKey : process.env.SECRET_ACCESS_KEY });
private params = {
Body: null || Object,
Bucket: "",
Key: ""
};
public send(stream : any) {
this.params.Body = stream;
return this;
}
public toBucket(bucket : string) {
this.params.Bucket = bucket;
return this;
}
public toFolder(path : string) {
this.params.Key = path;
return this;
}
public upload() {
return new Promise((resolve, reject) => {
if (process.env.ACCESS_KEY_ID == null || process.env.SECRET_ACCESS_KEY == null) {
return reject("ERR_NO_AWS_CREDENTIALS");
}
this.s3.upload(this.params, (error : any, data : any) => {
return error ? reject(error) : resolve(data);
});
});
}
}

Answer 1

Sinon fake servers are something you might use to develop a client that itself makes requests, instead of a wrapper around an existing client like AWS.S3, like you're doing. In this case, you're better off just stubbing the behavior of AWS.S3 instead of testing the actual requests it makes. That way you can avoid testing the implementation details of AWS.S3.
Since you're using TypeScript and you've made your s3 client private, you're going to need to make some changes to expose it to your tests. Otherwise, you won't be able to stub its methods without the TS compiler complaining about it. You also won't be able to write assertions using the params object, for similar reasons.
Since I don't use TS regularly, I'm not too familiar with it's common dependency injection techniques, but one thing you could do is add optional constructor arguments to your S3Uploader class that can overwrite the default s3 and arguments properties, like so:
constructor(s3, params) {
if (s3) this.s3 = s3;
if (params) this.params = params;
}
After which, you can create a stub instance and pass it to your test instance like this:
const s3 = sinon.createStubInstance(AWS.S3);
const params = { foo: 'bar' };
const uploader = new S3Uploader(s3, params);
Once you have the stub instance in place, you can write assertions to make sure the upload method was called the way you want it to be:
sinon.assert.calledOnce(s3.upload);
sinon.assert.calledWith(s3.upload, sinon.match.same(params), sinon.match.func);
You can also affect the behavior the upload method using the sinon stub api. For example, to make it fail like so:
s3.upload.callsArgWith(1, null);
Or make it succeed like so:
const data = { whatever: 'data', you: 'want' };
s3.upload.callsArgWith(1, null, data);
You'll probably want a completely separate test for each of these cases, using an instance before hook to avoid duplicating the common setup stuff. Testing for success will involve simply awaiting the promise and checking that its result is the data. Testing for failure will involve a try/catch that ensures the promise was rejected with the proper error.
Also, since you seem to be doing actual unit tests here, I'll recommend testing each S3Uploader method separately instead of calling them all in once big test. This drastically reduces the number of possible cases you need to cover, making your tests a lot more straightforward. Something like this:
#suite
class S3UploaderTest {
params: any; // Not sure the best way to type this.
s3: any; // Same. Sorry, not too experienced with TS.
uploader: S3Uploader | undefined;
before() {
this.params = {};
this.s3 = sinon.createStubInstance(AWS.S3);
this.uploader = new S3Uploader(this.s3, this.params);
}
#test
"send should set Body param and return instance"() {
const stream = "HalloWelt";
const result = this.uploader.send(stream);
Chai.expect(this.params.Body).to.equal(stream);
Chai.expect(result).to.equal(this.uploader);
}
#test
"toBucket should set Bucket param and return instance"() {
const bucket = "onetimeupload.test"
const result = this.uploader.toBucket(bucket);
Chai.expect(this.params.Bucket).to.equal(bucket);
Chai.expect(result).to.equal(this.uploader);
}
#test
"toFolder should set Key param and return instance"() {
const path = "onetimeupload.test"
const result = this.uploader.toFolder(path);
Chai.expect(this.params.Key).to.equal(path);
Chai.expect(result).to.equal(this.uploader);
}
#test
"upload should attempt upload to s3"() {
this.uploader.upload();
sinon.assert.calledOnce(this.s3.upload);
sinon.assert.calledWith(
this.s3.upload,
sinon.match.same(this.params),
sinon.match.func
);
}
#test
async "upload should resolve with response if successful"() {
const data = { foo: 'bar' };
s3.upload.callsArgWith(1, null, data);
const result = await this.uploader.upload();
Chai.expect(result).to.equal(data);
}
#test
async "upload should reject with error if not"() {
const error = new Error('Test Error');
s3.upload.callsArgWith(1, error, null);
try {
await this.uploader.upload();
throw new Error('Promise should have rejected.');
} catch(err) {
Chai.expect(err).to.equal(err);
}
}
}
If I were doing this with mocha proper, I'd group each method's tests into a nested describe block. I'm not sure if that's encouraged or even possible with mocha-typescript, but if so you might consider it.