React developer

1. Declaration
Functional components are nothing but JavaScript functions and therefore can be declared using an arrow function or the function keyword. Class components, on the other hand, are declared using the ES6 class. 
2. Handling props
In functional components, the handling of props is pretty straightforward. Any prop provided as an argument to a functional component can be directly used inside HTML elements. this keyword is used in the case of class components.
3. Handling state
Functional components use React hooks to handle state. It uses the useState hook to set the state of a variable inside the component. We cannot use React Hooks inside class components, therefore state handling is done very differently in a class component.

Props is the shorthand for Properties in React. They are read-only components which must be kept pure i.e. immutable. They are always passed down from the parent to the child components throughout the application. A child component can never send a prop back to the parent component. This help in maintaining the unidirectional data flow and are generally used to render the dynamically generated data.

The useState() is a built-in React Hook that allows you for having state variables in functional components. It should be used when the DOM has something that is dynamically manipulating/controlling. The idea with the usage of hooks is that we will be able to keep our code more functional and avoid class-based components if they are not required.

A Custom Hook is a function in Javascript whose name begins with ‘use’ and which calls other hooks. It is a part of React v16.8 hook update and permits you for reusing the stateful logic without any need for component hierarchy restructuring.
In almost all of the cases, custom hooks are considered to be sufficient for replacing render props and HoCs (Higher-Order components) and reducing the amount of nesting required. Custom Hooks will allow you for avoiding multiple layers of abstraction or wrapper hell that might come along with Render Props and HoCs.
The disadvantage of Custom Hooks is it cannot be used inside of the classes.

Hoisting is a default behaviour of javascript where all the variable and function declarations are moved on top.
This means that irrespective of where the variables and functions are declared, they are moved on top of the scope. The scope can be both local and global.

Example 1:
hoistedVariable = 3;
console.log(hoistedVariable); // outputs 3 even when the variable is declared after it is initialized 
var hoistedVariable;

No, the const variable doesn't make the value immutable. But it disallows subsequent assignments(i.e, You can declare with assignment but can't assign another value later)

const userList = [];
userList.push('Dev); // Can mutate even though it can't re-assign
console.log(userList);

Promise object has four states -

Pending - Initial state of promise. This state represents that the promise has neither been fulfilled nor been rejected, it is in the pending state.
Fulfilled - This state represents that the promise has been fulfilled, meaning the async operation is completed.
Rejected - This state represents that the promise has been rejected for some reason, meaning the async operation has failed.
Settled - This state represents that the promise has been either rejected or fulfilled.

Tip 1 : The cross questioning can go intense some time, think before you speak.

Tip 2 : Be open minded and answer whatever you are thinking, in these rounds I feel it is important to have opinion.

Tip 3 : Context of questions can be switched, pay attention to the details. It is okay to ask questions in these round, like what are the projects currently the company is investing, which team you are mentoring. How all is the work environment etc.