[CCI] Update Developer Guide

CHANGELOG.md

@@ -5,6 +5,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Added
 - Add release details to releasing.md ([319](https://github.com/opensearch-project/opensearch-js/pull/319))
 - Added Amazon OpenSearch Serverless in user_guide ([#372](https://github.com/opensearch-project/opensearch-js/issues/372))
+- Add guidelines on installing yarn, dependencies; instructions on running ESLint in developer_guide ([#439](https://github.com/opensearch-project/opensearch-js/issues/435))
 ### Dependencies
 - Bumps `aws4` from 1.11.0 to 1.12.0
 - Bumps `minimist` from 1.2.6 to 1.2.8
@@ -22,6 +23,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Changed
 - Remove test artifacts from gh_pages workflow ([#335](https://github.com/opensearch-project/opensearch-js/issues/335))
 - Make fields in `BulkOperation` optional to match OpenSearch Bulk API requirements ([#378](https://github.com/opensearch-project/opensearch-js/pull/378))
+- Remove guidance on using npm and switch completely to yarn in developer_guide ([#439](https://github.com/opensearch-project/opensearch-js/issues/435))
 ### Deprecated
 ### Removed
 ### Fixed
@@ -80,4 +82,4 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 [2.1]: https://github.com/opensearch-project/opensearch-js/releases/tag/2.1.0
 [2.2]: https://github.com/opensearch-project/opensearch-js/releases/tag/2.2.0
-[Unreleased]: https://github.com/opensearch-project/opensearch-js/compare/2.2...HEAD
+[Unreleased]: https://github.com/opensearch-project/opensearch-js/compare/2.2...HEAD

DEVELOPER_GUIDE.md

@@ -3,7 +3,10 @@
     - [Git Clone OpenSearch Node.js Client Repository](#git-clone-opensearch-nodejs-client-repository)
     - [Install Prerequisites](#install-prerequisites)
       - [Node.js](#nodejs)
+      - [Yarn](#yarn)
       - [Docker](#docker)
+    - [Installing Dependencies](#installing-dependencies)
+    - [Using ESLint](#using-eslint)
     - [Unit Testing](#unit-testing)
     - [Integration Testing](#integration-testing)
       - [Execute integration tests from your terminal](#execute-integration-tests-from-your-terminal)
@@ -25,30 +28,63 @@ e.g. `git clone https://github.com/[your username]/opensearch-js.git`.
 
 The minimum supported version of Node.js is v10.
 
+#### Yarn
+
+This project uses yarn as it's package manager. You can use npm to install yarn:
+
+```
+npm install --global yarn
+```
+
 #### Docker
 
 [Docker](https://docs.docker.com/install/) is required for building some OpenSearch artifacts and executing integration tests.
 
+### Installing Dependencies
+
+To install all the dependencies needed for this project, run
+
+```
+yarn install
+```
+
+### Using ESLint
+
+To do a ESLint check on the project, run:
+
+```
+yarn run lint
+```
+
+To let ESLint to fix errors in your code automatically, run:
+
+```
+yarn run lint:fix
+```
+
+This will instruct ESLint to try to fix as many issues as possible. The fixes are made to the actual files themselves and only the remaining unfixed issues are output.
+
+Not all problems are fixable using this option, and some of them you will need to fix manually.
+
 ### Unit Testing
 
 Go to your terminal and run:
 
 ```
 cd folder-path
-npm run test:unit
+yarn run test:unit
 ```
 
-To run a specific unit test, you can use the following jest command. `npm test` will not work.
+To run a specific unit test, you can use the following jest command. `yarn test` will not work.
 
 ```
 cd folder-path/to/test
 jest TestName
 ```
 
-If you don't have jest, you can install it via npm or yarn
+If you don't have jest, you can install it via yarn
 
 ```
-npm i -g jest-cli
 yarn global add jest-cli
 ```
 
@@ -69,7 +105,7 @@ If you have not installed docker-compose, you can install it from this [link](ht
 
 2. Run all integration tests.
    ```
-   npm run test:integration:helpers
+   yarn run test:integration:helpers
    ```
 3. Stop and clean containers.
    ```