Basic Questions Related to HTML/CSS Intermediate Questions on React Js ( Hooks , Pure Components , Lifecycle methods , Optimizations techniques , VDOM and how it works ) Intermediate - Difficult Questions Related to NodeJs ( Event loop , Security measures , Optimizations of API , JWT process , Middleware's and its implementations , Multi threading using nodejs ) MongoDB - Diff btw NoSQL and SQL , Indexes , Few Aggregations methods AWS - Deployments , S3 , Lambda workflow Coding questions : Custom Implementation of Array flat , Javascript Objects joining with a scenario based question with O(n) Implementation

What is a Ref in React and what are they used for?

Cannot disclose company policy. They were mostly on React, HTML5,CSS3.