JavaScript

Question 1

What is Javascript?

Answer 1

A high-level interpreted programming language that allows developers to add behavior to web pages

Question 2

Let vs Const

Answer 2

let allows reassignment

const does not allow reassignment after declaration

var is no longer used

Question 3

What is the difference between == & === ?

Answer 3

Quality operators.

== checks for equality regardless of type

=== checks for value and type equality

Question 4

Explain prototypal inheritance

Answer 4

When we read a property from an object, and it’s missing, JS takes it from the hidden property - prototype

Example:
let animal = {eats: true};
let rabbit = {jumps: true};

rabbit.__proto__ = animal

alert( rabbit.eats ); //true
alert( rabbit.jumps ); //true

Question 5

Explain closure

Answer 5

The combination of a function bundled together (enclosed) with references to its surrounding state (lexical environment)

Closure gives you access to an outer function’s scope from an inner function

Question 6

Explain event delegation

Answer 6

A single event listener is attached to a common ancestor, and events are handled based on the target element

Question 7

Explain the Event Loop

Answer 7

A mechanism that handles async operations by queuing them up, and executing them in the order they are queued

This keeps the main thread free to handle user interactions

Question 8

Explain the bind method

Answer 8

A method used so that an object can borrow a method from another object

For example, if an object has a function that another object needs to use, you can bind it instead of duplicating functionality

Question 9

Explain asynchronous programming

Answer 9

Allows the program to perform other tasks while waiting for async operations to complete.

Uses mechanisms like callbacks (older), promises, and async/await

Question 10

Callback vs Promise

Answer 10

Callback: functions passed as args to other functions, and are executed after a task is completed.
* Can pass data from parent to child
* Runs after parent completes

Promise: objects representing the eventual completion or failure of an async operation.
* fulfilled - success
* rejected - failure
* pending - not fulfilled/rejected
* settled - fulfilled or rejected

Question 11

Explain hoisting

Answer 11

Variable and function declarations are moved to the top of their scope during compilation

Allows them to be used before they’re declared

Question 12

Explain the this keyword

Answer 12

Refers to the current execution context

Used to access or modify the props of the current object

Question 13

Arrow vs Regular Functions

Answer 13

Arrow:
* simple, shorter, can skip the return keyword
* do not use ‘this’ or ‘arguments’
* cannot be used as constructors
const multiply = (num1, num2) => num1 * num2

Regular:
* have more flexibility
* can be used in more scenarios

Question 14

Explain array.map

Answer 14

Creates a new array by applying a provided function to each element of an existing array

Question 15

Explain array.filter

Answer 15

Creates a new array with elements that pass a specific test implemented by the provided function

Question 16

Explain array.forEach

Answer 16

iterates over elements of an array and executes a provided function once for each element

Does not create a new array

Question 17

Explain array.reduce

Answer 17

Reduces elements of an array to a single value.

Takes a callback function and an optional initial value, then the callback is applied to each array element, accumulating the result.

Question 18

Explain debouncing

Answer 18

Used to optimize event handling by delaying the execution of a function for a specified period of time, or period of inactivity

Question 19

Explain throttling

Answer 19

Used to optimize event handling by limiting how often a function can be called in a period of time

Question 20

Explain error handling

Answer 20

Errors are handled using try/catch blocks.

The code that may throw an error is contained in the try block, and the error handling is contained in the catch block

Question 21

Explain local storage

Answer 21

A web storage object for storing key value pairs locally on the client’s browser

It persists after the browser is closed

Question 22

Explain session storage

Answer 22

A web storage object for storing key value pairs locally on the client’s browser

It clears after a session ends (i.e. a browser is closed)

Question 23

Explain CORS, and ways to mitigate it

Answer 23

A mechanism that supports secure requests and data transfers from outside origins (domain, scheme, or port)

For example, when using fonts hosted from another website.

Prevents unauthorized cross-origin requests

Question 24

null vs undefined

Answer 24

null represents the intentional absence of a value

undefined is assigned to variables that have been declared but not yet assigned a value

Question 25

Explain Promise.all

Answer 25

takes an iterable of promises as input

returns a single promise that resolves when all the promises in the iterable have resolved,

or rejects when any one of the promises is rejected

Question 26

Explain prototypes

Answer 26

Objects can be linked to other objects, forming a prototype chain.

When a property or method is accessed on an object, the JavaScript engine first checks if that property or method exists on the object itself.

If it does not, it will check the object’s prototype, and so on, until it reaches the end of the prototype chain.

If the property or method is not found, it will return undefined.

Question 27

Explain ES6 Classes

Answer 27

The class keyword provides a more familiar and clean syntax for creating objects and their methods

but under the hood, it still uses prototypes to link objects and inherit properties and methods.

Question 28

Explain a ‘pure’ function

Answer 28

A function that, given the same input, always returns the same output with no side effects

It does not modify or rely on state

Question 29

Explain …spread

Answer 29

allows an iterable, such as an array or string, to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected

• combining arrays
  *adding a new value to the front of an array
• updating an object

Question 30

Explain async/await

Answer 30

async defines an asynchronous function

await is used inside the async function to wait for the resolution of a promise before proceeding to the next line

Question 31

Explain how fetch works for HTTP requests

Answer 31

fetch makes an http request, and returns a promise that resolves to the response to that request

Question 32

Explain setInterval() and setTimeout()

Answer 32

functions for scheduling the execution of a function later in time

setInterval calls a func repeatedly with a fixed time interval

setTimeout calls a func after a fixed time delay

Question 33

Explain Same-Origin policy

Answer 33

A security measure implemented by web browsers to restrict web pages from making requests to a different domain

Question 34

Explain Object.create()

Answer 34

a method used to create a new obj with the specified prototype obj and optional props

enables prototype-based inheritance

Question 35

Explain cookies

Answer 35

Data stored in small text files, used to remember information about a user

Useful for remembering user names, emails, etc.

By default they’re deleted when the browser is closed

Question 36

Explain the addEventListener method

Answer 36

attaches an event handler to a specified element, which executes when the event occurs

Question 37

Explain memoization

Answer 37

Storing the results of expensive function calls when the same inputs occur again

Improves the performance of recursive functions by avoiding redundant computations

Question 38

Explain the typeOf operator

Answer 38

determines the type of a variable, returns type as a string

“number,” “string,” “boolean,” “object,” “function,” or “undefined.”

Question 39

Explain the Symbol data type

Answer 39

Creates unique, immutable (unchanging) values that can be used as object properties

Often used as keys in objects to avoid naming conflicts

Question 40

Function declaration vs Function expression

Answer 40

Declarations
standalone statements, can be hoisted - used before they’re declared

Expressions
can be anonymous or assigned to variables, can be passed as arguments, not hoisted

Question 41

How do you handle CORS related issues?

Answer 41

Configure the server to include the appropriate CORS headers

Access-Control-Allow-Origin

Question 42

Explain currying

Answer 42

When a func with multiple args is transformed into a sequence of funcs, each taking a single arg

Allows partial application of the fun, creating new funcs with fewer args

Question 43

Explain Object.keys

Answer 43

Returns an array of a given object’s own string-keyed property names

Question 44

Explain WeakMap and WeakSet

Answer 44

ES6 collections

Allow devs to create a mapping of key-value pairs, or set of unique values without preventing the garbage collection of keys

Question 45

Shallow vs Deep copying

Answer 45

Both
* create a new obj or array

Shallow
* copies refs of the original obj’s properties or array elements
*may share refs

Deep
*recursively copies all nested objects or arrays
* creates new refs

Question 46

Explain isNaN and Number.isNaN

Answer 46

Both
* determine if a value is the number value, NaN

Number.isNaN (ES6)
* true if the given value is a number with value NaN. Otherwise, false.

isNaN
* true if the given value is NaN after being converted to a number; otherwise, false.

Question 47

Explain the module pattern

Answer 47

Design pattern that encapsulates private and public members within a module.

Typically uses closures to create a private scope for variables and functions, exposing only what is necessary to the outside world.

Question 48

Primitive data types

Answer 48

undefined
null
boolean
number
string
symbol

Question 49

Truthy vs Falsy values

Answer 49

Falsy values: false,
0, ‘ ‘ empty string, null, undefined, NaN

Everything else is truthy

Question 50

Explain Infinity

Answer 50

a numeric value representing positive infinity

represents values that exceed the upper limit of the floating-point

Question 51

How do you convert a string to a number

Answer 51

3 ways:
* parseInt - extracts the whole number
*parseFloat - extracts the whole number and decimals
*unary plus operator (+)

Question 52

Explain type coercion

Answer 52

Automatic conversion of values from one data type to another

*comparing values of different types
*using operators

Question 53

Explain string interpolation

Answer 53

The process of evaluating variables and expressions within a string literal
(${someString})