feat: doc testing added (#84)

* feat: doc testing added

* fix: redundant section removed

* Update doc/TESTING.md

Co-authored-by: r-l-d <ross.diliegro@accenture.com>

* Update doc/TESTING.md

Co-authored-by: r-l-d <ross.diliegro@accenture.com>

* Update doc/TESTING.md

Co-authored-by: r-l-d <ross.diliegro@accenture.com>

* Update doc/TESTING.md

Co-authored-by: r-l-d <ross.diliegro@accenture.com>

* fix review comments

* amend to fix review comments

---------

Co-authored-by: r-l-d <ross.diliegro@accenture.com>

doc/TESTING.md

@@ -1,22 +1,97 @@
 # Testing
-This chapter should provide the user with all needed information around testing within your project.
 
 ## Content
-- [Different kind of tests](#different-kind-of-tests)
-- [Tooling](#tooling)
+- [Different Test Types](#different-test-types)
+- [Testing Strategy](#testing-strategy)
+- [TDD (Test-driven development)](#tdd-test-driven-development)
+- [Tooling - Testing Libraries Installed](#tooling---testing-libraries-installed)
+- [How to install Jest](#how-to-install-jest)
 - [How to write tests](#how-to-write-tests)
 - [How to run tests](#how-to-run-tests)
 
-## Different kind of tests
-This section should display all the different kinds of tests that you write in your project. Explain what each kind of test is for and why you use them in your project. You can also give an outlook on new tests that should be added in the future of your project. If you have a QA in place, also mention what kind of tests they perform and add contact persons.
+## Different Test Types
+In this project, Unit Testing is used to ensure that a section of an application (known as the "unit") meets its intended goal.
 
+Unit Testing - The main objective of unit testing is to isolate written code to test and determine if it works as intended. 
 
-## Tooling
-Please explain what kind of tools you use for testing, why you have choosen them and how to work with them.
+In future other testing types like Integration Testing, Performance Testing can be added in the Continuous Integration process.
 
+## Testing Strategy 
+The tests are written to verify whether the software works as intended and to catch bugs and errors before they pushed and make it into production. Testing helps us catch issues early in the development process.
+
+The goal is to write automated unit tests for each module or function in the codebase to ensure that it works as intended and the changes do not introduce regressions. The minimum code coverage of unit tests should be **70%**. To achieve this goal, we follow a comprehensive testing strategy of TDD (Test-driven development) process.
+
+## TDD (Test-driven development)
+**telemetry-functions** follows TDD (Test-driven development) process, a method of implementing software programming that interlaces unit testing, programming and refactoring on source code, to eusure the quality, reliability, and functionality of the software.
+
+Test-driven development (TDD) is a software development approach where developers write automated tests before writing the code. The idea is to write a test case that fails initially and then write the code to make the test pass. The process is repeated for each new feature or change to the codebase.
+
+## Tooling - Testing Libraries Installed
+**Jest** - a JavaScript testing framework is testing library used to write the test cases,  It allows you to write tests with an approachable, familiar and feature-rich API that gives you results quickly
+
+## How to install Jest
+Install the jest package (and optional typings) to the existing project's package.json by the below command
+
+`npm install --save-dev jest @types/jest`
+
+When cloning the project, the package.json has the library in dev dependency which installs Jest with `npm install` in the project's root directory.
 
 ## How to write tests
-This section should explain how to write different tests on a basic level. Please also include information about your testing strategies. Don't hesitate to use a lot of code exaples and links to further information and external tech documentation if you use .
+
+Jest is configured by default to look for .js, .jsx, .ts and .tsx files inside of __tests__ folders, as well as any files with a suffix of .test or .spec (this includes files called test or spec).
+
+The project has  __tests__ folders which have *.test.ts files that will contain the test cases and fixtures folder having all the payload mocks used in test cases.
+
+To write a test case we first define the outcomes that we must validate to ensure that the functionality is working correctly.
+
+- To group test cases in Jest into a test suite we can use the `describe()` function. It takes a suite name (string) and a handler function as the first two arguments.
+ - In Jest we use the `it()` function. It takes a test name (string) and a handler function as the first two arguments.
+ - An expectation is created with the `expect()` function. It returns an object of matcher methods `toMatchObject()` with which we assert something expected about the tested value.
+
+Example: A test to check if the function returns "foo" for multiples of 3
+```
+// fooBar.test.ts
+const fooBar = require("./fooBar");
+
+describe("fooBar", () => {
+  it("returns 'foo' for multiples of 3", () => {
+    expect(fooBar(3)).toMatch("foo");
+  });
+});
+```
 
 ## How to run tests
-Here you should provide a guide on how to run the different tests you use. Write down the exact commands to use and where to execute them. You could also use the structure of your set up step-by-step guide from your Contribute page.
+To run tests with Jest call the jest command inside the root of the project folder.
+
+The project's package.json is updated with the  following test script that calls the jest command:
+```
+{
+    // ... package.json contents
+    "scripts": {
+                // ... existing scripts
+                "test": "jest",
+                "test:coverage": "jest --coverage",
+                "test:dev": "jest --watch",
+    }
+}
+```
+
+To run the Jest testing library
+
+**_`npm run test`_**
+
+In this case, `npm run test` is defined as jest in the package.json scripts, which will execute the Jest testing library
+
+To run the test in development environment
+
+**_`npm run test:dev`_**
+
+This is defined as `jest --watch`, which will execute Jest in "watch" mode.
+
+When you run npm run test without any arguments, Jest will run all the test suites once and then exit. However, when you run npm run test:dev, Jest will enter "watch" mode and continuously watch for changes to your test files or the code being tested. When a file changes, Jest will re-run the affected tests. This makes it faster and more convenient to develop and test your code.
+
+To run the test and to create a test coverage report
+
+**_`npm run test:coverage`_**
+
+Also note that you can pass additional arguments to Jest using `--` followed by the argument(s) you want to pass. For example, you could run `npm run test  --coverage` to generate a test coverage report.