Interview Questions

Question 1

What is the DOM?

Answer 1

DOM stands for Document Object Model. It is a representation of the HTML document displayed in a window. Objects are organized in a hierarchy.

Question 2

What is ECMAScript and how does it relate to JavaScript?

Answer 2

ECMA is a standardized specification for scripting languages; JavaScript is a scripting language based on ECMA specification.

Question 3

What is JavaScript?

Answer 3

JavaScript is a programming/scripting language that allows you to implement complex logic on web pages;

JavaScript is lightweight and interpreted programming language. It is most commonly utilized by client-side technologies.

Question 4

What are some advantages that can be gained through using JavaScript

Answer 4

Less server interaction - Allows for client-side validation of user-input, reducing the number of unsuccessful HTTP requests.

Increased Interactivity - The Document Object Model can be manipulated based on different events initiated by the user

Question 5

What does it mean that JavaScript is loosely typed?

Answer 5

This means that JavaScript does not require you to specify a type when declaring a variable.

Question 6

What does it mean that JavaScript is an interpreted language?

Answer 6

This means that JavaScript is not pre-compiled. It is read line by line by the browser.

Question 7

What tag would you use to wrap around a block of JavaScript on an HTML document and where would you need to place it?

Answer 7

Use the tag to wrap the JavaScript statement, and make sure the block is nested within the body of the HTML document.

Question 8

Is JavaScript case sensitive?

Answer 8

Yes

Question 9

How do you insert a comment in JavaScript?

Answer 9

// - Single-Line comment
/*
* Multi-line comment
*/

Question 10

Does Javascript have data types? If it does, what are they?

Answer 10

Yes, and they are Boolean, Null, Undefined, Number, String, Object, Symbol, and Function

Question 11

What are three different ways you can create variables in Javascript and when would you use each?

Answer 11

var - scope of the variable is its current execution context
let - scope of the variable is limited to the block in which it is created
const - scope of const is similar to let statement, but once assigned a value, it cannot be changed

Question 12

What are global variables? How are these variables declared and what are the problems associated with using them?

Answer 12

Global variables are those that are available throughout the length of the code.

Global variables are created when the variables are declared outside of any block

Global variables make it hard to debug and test code

Question 13

What Javascript values are considered falsey? What are considered truthy?

Answer 13

Falsey Values:

• undefined
• null
• NaN
• false
• ” “(empty string)
• 0

everything else is truthy

Question 14

What are undeclared and undefined variables?

Answer 14

Undeclared variables are those that do not exist in a program, and if called gives a “reference not found” error

Undefined variables are those that are declared but have not been initialized with a value, and if called gives a “undefined” error

Question 15

What is === operator vs == operator?

Answer 15

== double equals will perform a type conversion;
=== triple equals will not perform a type conversion, and will return false if the types are different

Question 16

What is the typeof operator?

Answer 16

an operator that returns a string indicating the type of the unevaluated operand

Question 17

What is hoisting in Javascript?

Answer 17

a Javascript mechanism where variables and function declarations are moved to the top of their scope before code execution

Question 18

What are anonymous functions?

Answer 18

Anonymous functions are functions that are dynamically declared at runtime. They are not given a name in the same way as normal functions

Question 19

What is an IIFE?

Answer 19

an Immediately Invoked Function Expression that runs as soon as it is defined

Question 20

How can you add new elements dynamically?

Answer 20

Using functions like document.createElement() and specifying a DOM tag. You can also use appendChild() on that element to add a Node beneath your newly created element

Question 21

Explain how you can submit a form using Javascript?

Answer 21

To submit a form using Javascript use document.getElementById(“myform”).submit()

Question 22

What is an event?

Answer 22

an event is an action that takes place on the DOM, like mouse click or tapping keyboard, etc

Question 23

What are some common Javascript events?

Answer 23

onchange, onclick, onmouseover, onmouseout, onload, etc

Question 24

What is an event listener?

Answer 24

Event listeners are procedures or functions that wait for an event to occur, method ex: addEventListener

Question 25

What is JSON?

Answer 25

Javascript Object Notation. A safe and reliable data interchange format in Javascript, which is easy to understand for both users and machines

Question 26

What is Ajax?

Answer 26

an abbreviation of Asynchronous Javascript and XML. Uses the XMLHttpRequest object to send and receive information with the server in formats up-to and including JSON, XML, HTML, and text files.

Question 27

What are advantages of Ajax?

Answer 27

• Faster retrieval of data because its asynchronous
• Reduced Bandwidth usage and increased speed
• Improved User Experience

Question 28

What are disadvantages of Ajax?

Answer 28

• Browser Incompatibility
• Ajax is dependent on Javascript. If there’s a Javascript problem with the browser, Ajax will not be supported
• slow and unreliable network connection

Question 29

What is the for-in loop in Javascript?

Answer 29

this loop iterates over the enumerable properties of an object.

Question 30

What is the for-of loop in Javascript?

Answer 30

this loop iterates over iterable objects, like Strings and Arrays

Question 31

What is the use of isNaN function?

Answer 31

this function returns true if the argument is not a number otherwise it is false

Question 32

What is negative infinity?

Answer 32

Negative Infinity is a number in Javascript which can be derived by dividing negative number by zero

Question 33

What are of the rules that Javascript variables names must conform to?

Answer 33

Cannot delare a variable name that is a reserved Javascript keyword. Names must begin with a letter or an underscrore

Question 34

Explain the working of timers in Javascript? Also discuss the drawbacks of using the timer, if any?

Answer 34

• Timers are used to execute a piece of code at a set time or also to repeat the code in a given interval of time. functions include setTimeout, setInterval, and clearInterval
• Timers are operated within a single thread, and thus events might queue up, waiting to be executed

Question 35

What does JSON.parse() do?

Answer 35

it turns a Javascript string formatted as JSON into a Javascript object

Question 36

What does JSON.stringify do?

Answer 36

To send a javascript object as a json, we need to turn it into a string.