Version number: 5b8d0fd276b6d288905ed2f63a934e057e8feca2

How to work with app

Description

EE simple application

Installation

$ npm install

Running the app

# development
$ npm run start

# watch mode
$ npm run start:dev

# production mode
$ npm run start:prod

Test

# unit tests
$ npm run test

# e2e tests
$ npm run test:e2e

# test coverage
$ npm run test:cov

Production deployment

On production .env file should be created based on .test.env

How to use

The calculator is available after starting under http://localhost:3000/calculator. You should send a POST with a the folowing body: { "equation": "2+3" }

Analyzes

I spent in the meantime about 0,5h before starting the coding on the project analysis and did below notes.

Note about last approach

I did a first try to the exercise here: https://github.com/piotrduszynski-besolve/ee-calculator

I decided to try again with a different approach, as I spend to much time on setup the project tech stack. I decided to build a project based on nestjs, which speeds up the development phase significantly, thanks to build-in generators for setup projects and project files.

Product Analyzes

Business requirements

Product requirements

• Calculator should be attractive
  • ASCI Art in JSON
  • Web calculator
• Equal Expert Branding
  • Branding in JSON
  • Image
• Feature expected from the simple pocket calculator:
  • sum, subtract, divide, multiplication
  • order of operations execution
  • easy way to input an equation
  • integer and floating
  • 10? chars of precision(is there a limitation for number?)
  • clear input

Technical Requirements

• coding exercise no more than 90 minutes, code like for the paying client - what compromises I would take.

  I decided to split time into 15 minutes sessions maximum. To keep on track of what progress I made and do I need to change a plan o execution.

• Required: Test Coverage, Simplicity, Self-explanatory code

• Include Readme

• Include the Version number

How to deliver business value quickly

• Calculation backend endpoint in the first stage
• 3rd party calc - https://mathjs.org/, https://nerdamer.com/ , https://www.npmjs.com/package/static-eval
• eval could be used to calc equation from a string, but executing JavaScript from a string can lead to an enormous security risk.
• ASCI Art -> Equal Expert Branding
• Server rendered calc webpage
• nestjs as framework - easy and quick to use, embedded server rendering, build in generator for project and files

Risks

• Small amount of time -> 15 minutes coding sessions
• The small amount of time can lead to problems with the implementation of 2 important tasks(in my opinion):
  • deployment
  • product analytics/metrics

Story Mapping - plan of work

ID	Input Equation	Calculate	Display Result	Memorize Equation	Clear
1	Endpoint to send equation	Basic operations(+,-)	Add branding - JSON
1.2	Validate input string + trim		Add branding - ASCI art
3		Basic operations(*,/)
3.1		Order of operations
3.2		Divide by 0
4			HTML page + branding image
5	Home page with equation input

Technology stack

CI

• husky + nestjs
• Create app artifacts using Github Releases
• When MVP is created, think about adding Github action for CI
• When MVP is created, think about the creation of an artifact as a Docker image in GitHub Container Registry

CD

• Manual release to AWS Lightsail
• When MVP is created, add deployment via AWS Pipeline
• When MVP is created, add IaC

Product Monitoring

• When MVP is created, add Mixpanel

Session Notes

1 Add Nestjs + Husky(4 minutes)

• The same functionalities as with manual initialization from the last approach + TS + Prittier and more

2 Added equation input (13 minutes)

• I generated controller and interfaces for calculator data - a quick one to do

3 Added Json Branding (14 minutes)

• I needed to read about handling configuration in nestjs

4 Fixed E2E Test (4 minutes)

5 Sum and minus operation(16 minutes)

• I wanted to write calc from scratch, but I realized it's risky in 90 minutes to deliver product value for you.
• Decided to use Mathjs
• Updated stories

6 Equation validator + calculation service(35 minutes)

• I had a problem with mocking. I decided to use a different approach with module providers
• Problem with one line regex for the equation.
• Simple task changed in some refactoring and additional tasks.
• Equation input is returning a number. That should be changed to string.
• I'll decide to skip ASCII art implementation

7 Add multiplication and division(30 minutes)

• Some problems with tests that throws exception for Infinity.
• There was a need to add Infinity handling for dividing by zero. It's how it works in Math.js. I needed to read about it
• Concentration was not so good as at the beginning

Retrospective

• After about an hour I started to feel a degradation of concentration. It's not efficient to work this way for long-term development. I decided to split an exercise into sessions, and between them try to write notes and rethink the coding approach, which finally end up in 2h exercise on a high concentration level. On the other hand, sessions helped me to decide on how to progress and rethink the current approach.
• Preparing a plan(story mapping) beforehand was the bull's eye. It gave me the ability to focus on development.
• Nestjs was a risky choice as I'm not very experienced in it. Any mistake or bigger problems could lead to time waste. However build-in generators save me a lot of time.
• Time was very short, so I think the code would require some refactoring(name changes, and maybe some simplifications)