Interview questions JavaScript

Question 1

Can you name two programming paradigms important for JavaScript app developers?

Answer 1

• JavaScript supports imperative/procedural programming along with Object-Oriented Programming and functional Programming.

JavaScript supports Object-Oriented Programming with prototypal inheritance.

Question 2

What is functional programming?

Answer 2

Functional programming produces programs by composing mathematical functions and avoids shared state & mutable data.

Question 3

What is the difference between classical and prototypal inheritance?

Answer 3

Classical Inheritance: instances inherit from classes and create sub-class relationships. Instances are typically instantiated via constructor functions with the new keyword. It may or may not use the class keyword. (Syntactic sugar)

Prototypal Inheritance:
Instances inherit directly from other objects. Instances are typically instantiated via factory functions or Object.create(). Instances may be composed from many different objects, allowing for easy selective inheritance.

Question 4

What are the pros of Object-oriented programming?

Answer 4

It’s easy to understand the basic concept of objects and easy to interpret the meaning of method calls. Tends to use an imperative style rather than a declarative style.

Question 5

What are the cons of Object-Oriented programming?

Answer 5

OOP typically depends on shared state. Objects and behaviors are typically tacked together on the same entity, which may be accessed at random by any number of functions in a non-deterministic order. May lead to undesirable behavior like race conditions.

Question 6

What are pros of functional programming?

Answer 6

• Avoid any shared state or side-effects. Eliminates bugs caused by multiple functions competing for same resources.
• Functions tend to be radically simplified, easily recomposed.
• Tends to favor declarative and denotational styles.

Question 7

What are cons of functional programming?

Answer 7

Over-exploitation can reduce readability.

More people are familiar with OO and imperative programming.

FP has a much steeper learning curve.

Question 8

When is classical inheritance an appropriate choice?

Answer 8

Never more than one level.

Favor object composition over class inheritance.

Question 9

when is prototypal inheritance an appropriate choice?

Answer 9

(More than one type)

• Delegation
• Concatenative
• Functional

All are equally useful in creating composition.

Question 10

What does ‘Favor object composition over class inheritance’ mean?

Answer 10

Code reuse should be achieved by assembling smaller units of functionality into new objects instead of inheriting from classes and creating object taxonomies.

Question 11

What are two-way data binding and one-way data flow, and how are they different?

Answer 11

Two-way data binding: UI fields are bound to model data dynamically such that when a UI field changes, the model data changes with it.

One-way data flow: The model is the single source of truth. Changes in the UI trigger messages that signal user intent to the model.

Only the model has the access to change the app’s state.

Question 12

What is a monolithic architecture?

Answer 12

Means your app is written as one cohesive unit of code whose components are designed to work together.

Question 13

What is a microservice architecture?

Answer 13

Your app is made up of lots of smaller, independent applications capable of running in their own memory space and scale independently.

Question 14

What are the pros of a monolithic architecture?

Answer 14

Many apps have cross-cutting concerns. When everything is running through the same app, it’s easy to hook up components to those cross-cutting concerns.

• There can also be performance advantages. shared-memory access is faster than inter-process communication.

Question 15

What are cons of a monolithic architecture?

Answer 15

They tend to get tightly coupled and entangled as the app evolves. It is difficult to isolate services for purposes such as independent scaling or code maintainability.

• difficult to understand.

Question 16

What are pros of a microservice architecture?

Answer 16

• Generally better organized since each microservice does a specific job. Can have performance advantages depending on how they’re organized.

Question 17

What are cons of a microservice architecture?

Answer 17

Unanticipated cross-cutting concerns.

Question 18

What is async, and why is it important?

Answer 18

Synchronous: Code executed from top to bottom.

Async: engine runs in an event loop.

Question 19

How does asynchronous programing work?

Answer 19

The entire engine runs in an event loop, and when a blocking operation is needed, the request is started, and the code keeps running without blocking for the result.

When response is ready, interrupt is fired, causing event handler to be run.

Question 20

What do bind(), apply(), and call() do?

Answer 20

They set this to a provided value, and also take in arguments.

Question 21

What are the differences between bind apply and call?

Answer 21

bind allows you to call a function later with a certain context.

Call and apply invoke the function immediately. Call accepts an argument list while apply accepts a single array of arguments.

Question 22

What is the use of Bind?

Answer 22

It allows us to inject a context into a function which returns a new function with updated context. this becomes a user-supplied variable, which is great for events.

Question 23

What are the three types of JavaScript scopes?

Answer 23

• Global Scope,
• Local/Function Scope
• Block Scope

Question 24

What is global scope?

Answer 24

What we normally do, when we declare variable globally.

Question 25

What is local/function scope?

Answer 25

When you declare a variable inside of a function.

Question 26

What is block scope?

Answer 26

When the variable’s scope is limited to a given block.

Question 27

What are the rules of the this keyword?

Answer 27

Global context - Refers to window object.

Local context - Refers to enclosing object. ‘Whenever a function is called by a preceding dot, that thing before the dot is this’

Whenever a constructor function is used, this refers to the specific instance of the object that is created and returned by the constructor function.

Whenever call or apply is used, this is explicitly defined.

Question 28

What are the ways of creating an object?

Answer 28

var obj = {};
var obj = new Object();

Question 29

How do you convert an object into a JSON string? How do you turn it back?

Answer 29

You convert a given object into a JSON string using JSON.stringify.

You turn it back with JSON.parse.

Question 30

How do you iterate over an object?

Answer 30

Either use a for in loop, or use object.keys

Question 31

What does object.values do?

Answer 31

Returns a list of values of an object.

Question 32

What does object.prototype do?

Answer 32

Provides a list of an object’s prototype, and all of the functions that exist on it.

Question 33

What does Object.prototype.hasOwnProperty do?

Answer 33

Helps to find out whether a given property/key exists in an object.

Question 34

What does Object.prototype.instanceof do?

Answer 34

-Evaluates whether a given object is the type of a particular prototype.

Question 35

What does Object.freeze do?

Answer 35

Allows us to freeze an object so that existing properties cannot be modified.

Question 36

What does Object.seal do?

Answer 36

It allows configurable properties but won’t allow new property addition or deletion of properties.

Question 37

What is prototypical inheritance?

Answer 37

Essentially setting properties on the object prototype, allowing you to create new objects that have those same properties.

Question 38

What are callbacks?

Answer 38

Callbacks are functions executed after an I/O operation is done.

Question 39

What are promises?

Answer 39

They are wrappers for callbacks which allow us to asynchronously code elegantly. Can control the flow of asynchronous operations.

Question 40

What are the two ways of creating regular expressions?

Answer 40

var reg = /ar/;
var reg = new RegExp('ar');

Question 41

What does map in javascript do?

Answer 41

We can get a new array by applying a transformation function on every element of that array.

Question 42

What does reduce do?

Answer 42

Reduces a given list to one final result. Cleaner than iterating.

Question 43

What does filter do?

Answer 43

processes each element in an array, and returns all of the ones that pass a test.

Question 44

How can we handle errors?

Answer 44

Use try… catch blocks, or if using promises have a rejection block.

Question 45

What is hoisting?

Answer 45

The process of pushing undeclared variables to the top of the program while running it.

Question 46

What is event bubbling?

Answer 46

A way of event propagation in the HTML DOM API when an event occurs in an element inside another element. Propagate upwards.

Question 47

What is event capturing?

Answer 47

The opposite of bubbling. Events propagate inwards instead of outwards.