You are mocking the db declared in the test file - not the saveUser that is actually used. The solution is to move the db declaration to its own module, for example: db.js

const AWS = require("aws-sdk");

const db = new AWS.DynamoDB.DocumentClient({
  apiVersion: "2012-08-10"
});

module.exports = db;

Then import it from the

module and test - this way we can mock the same db instance that saveUser is using.

I was able to successfully run the test using the following code:

Test code:

const sinon = require('sinon');
const { saveUser } = require('../userHandler');
const { expect } = require('chai');

const db = require('../db');

describe('user-name-handler', function() {
  afterEach(() => sinon.restore());

  it('Test saveUser() method', async function() {

      sinon.stub(db, 'put')
        .returns(new Promise((resolve, _) => resolve({
            statusCode: 201,
            body: 'some data'
      })));

      try {
          const result = await saveUser(db, 'Sample User', {
            log: () => {},
            exception: () => {}
          });

          expect(result).to.deep.equal({
            statusCode: 201,
            body: 'some data'
          });
      } catch (err) {
        console.log('err', err);
      }
  });
});

User handler file:

const db = require('./db');

const saveUser = (db, userName, logger) => {
  const Item = {
    id: uuid.v4(),
    userName,
    ttl: parseInt(Date.now() / 1000) + 900 // expire the name after 15 minutes from now
  };
  const params = {
    TableName: "my-table-name"
  };
  logger.log(`Saving new user name to DynamoDB: ${JSON.stringify(params)}`);

    return db.put(params, function(err, _) {
      if (err) {
        logger.exception(`Unable to connect to DynamoDB to create: ${err}`);
        return reject({
          statusCode: 404,
          err
        });
      } else {
        logger.log(`Saved data to DynamoDB: ${JSON.stringify(Item)}`);
        return resolve({
          statusCode: 201,
          body: Item
        });
      }
    });
}

module.exports = { saveUser };

package.json

{
  "name": "play",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "mocha --timeout 5000"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "aws-sdk": "^2.1373.0",
    "chai": "^4.3.7",
    "mocha": "^10.2.0",
    "sinon": "^15.0.4"
  }
}

Separate database connections in files

We can separate the database connection into a different file and import it into the handler implementation as well as the spec file.

db.js

import AWS from "aws-sdk";
const db = new AWS.DynamoDB.DocumentClient({ apiVersion: "2012-08-10" });

export default db;

yields() Function

The stub should not return a Promise directly, but should be chained with the arguments that .yields() and its callbacks will accept. We can change parameters to cover various branches of the code.

Code

describe("user-handler connection success", function () {
    const sandbox = sinon.createSandbox();
    afterEach(() => sandbox.restore());
    before(() => {
        sinon.stub(db, "put")
            .yields(null, true);
        sinon.stub(db, "get")
            .yields(null, { sampleKey: "sample value" });
        sinon.stub(db, "delete")
            .yields(null, { sampleKey: "sample value" });
    });
    after(() => {
        db.put.restore();
        db.get.restore();
        db.delete.restore();
    });

    it("Test saveUser() method success", async function () {
        const result = await userHandler.saveToken("sample user", {
            log: () => {},
            exception: () => {}
        });

        expect(result.statusCode).to.be.equal(201);
    });
});

Useful links

https://www.youtube.com/watch?v=vXDbmrh0xDQ