JavaScript

Question 1

What are functions and why are they referred to as first class functions?

Answer 1

Block of code that is self-contained
Optionally accept parameters and returns one value

Referred to as first class functions because they can be:

• Assigned to a value
• Passed as arguments
• Used as a return value

Question 2

What is the difference between var, let and const?

Answer 2

var

• Can be reassigned
• Global scope if declared outside of a function
• Hoisted to the top so it will throw undefined if you try to access them before

let

• Can be reassigned
• Scoped to what the closure is
• Throw reference error if you try to access them before

const

• Does not provide immutability but it ensures that the reference can’t be changed
• Scoped to what the closure is
• Throw reference error if you try to access them before

Question 3

What is prototypal inheritance?

Answer 3

Javascript has a baseline prototype of the object that everything inherits from and when you create a new object based on the other object, you can either inherit all those properties which will be default or you can overwrite them with your own properties

Question 4

What does “this” mean in JavaScript?

Answer 4

Depending on the scope, “this” can change values

If there is no scope, then it’s the global window object

Question 5

What is the data structure of the DOM?

Answer 5

Tree

Question 6

What is a stack?

What is a queue?

How would you create those data structures in JavaScript?

Answer 6

Stack is LIFO
Queue is FIFO
Use an array - pop/push, shift/unshift

Question 7

What is a worker and when would you use one?

Answer 7

Used in a browser to offload computationally expensive work to a different thread because JavaScript is single threaded so you don’t have any blocking operations

Question 8

What is event delegation and why is it important?

Answer 8

Traditionally, for event handlers in html, you could apply an event handler to every single element in your html

But event listeners are expensive because every time it renders, it’s checking if something has happened or not

Using event delegation, you can use one event listener at the top, and when that event listener is triggered, it bubbles up to the parent that handles the event

Important for performance reasons

Question 9

What is the difference between function expressions and arrow functions?

Answer 9

In function expressions, the this is bound to different values based on the context of which it is called

Arrow functions do not bind their own this, so it will lexically go up a scope and use the value of this in the parent scope

Arrow functions are a good choice for working with closures or callbacks but not a good choice for working with class/object methods or constructors

Question 10

What is the difference between continue and break statements?

Answer 10

Both affect loops

Continue statement terminates execution of the current iteration in a loop

Break statement stops the execution of a loop entirely

Question 11

What is the difference between return and yield statements?

Answer 11

Return statement is used to return the value of the function call to the caller and will not let you do anything else after the return statement

Yield statement is used to pause and resume a generator function

Question 12

What is the difference between a while loop and a for loop?

Answer 12

A while loop loops through a block of code as long as the condition evaluates to true (must ensure the condition will eventually return false otherwise stuck in an infinite loop)

A for loop loops through a block of code until the counter reaches a specified number

Question 13

What is the difference between an array and an object?

Answer 13

Both are considered special in Javascript
Both represent a data type that is mutable

Objects are used to store a collection of data and used to represent characteristics/properties, uses for/in loop to iterate over object properties

Arrays are used to store a list of values, uses for/of loop to iterate

Question 14

What is Javascript injection?

Answer 14

Injecting Javascript code which can modify a website’s design or hacking someone’s account credentials - can be prevented by sanitising user input

Not as serious as SQL injection because it is client-side and not as risky as XSS attack

Question 15

What is strict mode?

Answer 15

Allows the Javascript runtime to catch errors and other overlooked issues

Question 16

What does tree shaking mean?

Answer 16

Removing dead code from the bundle before you ship to your users

Reduces file size and loading time

Question 17

What is a polyfill?

Answer 17

A way to provide new functionality available in modern browsers or modern browser API to older browsers

Question 18

Immediately Invoked Function Expression

IIFE

Answer 18

Executes functions immediately

Useful because they don’t pollute the global scope - also a simple way to isolate variables and declarations

Wrapped inside parentheses to indicate a function expression rather than a function declaration

Question 19

What is scope?

Answer 19

Where the JS engine will look for things
Portion of the code where the variable is visible

Can be globally or locally defined:
- Global scope: highest level of scope
- Local scope: any scope defined past global scope and
broken down into function scope and block scope

Scope can be changed by using .call(), .apply(), .bind() methods

Question 20

What is a pure function?

Answer 20

Function that is idempotent and doesn’t produce side effects

Does not mutate shared state

Produce stronger guarantees of encapsulation than objects without function purity

Given the same input, a pure function will always produce the same output

Question 21

What is destructuring?

Answer 21

Simplified way of defining variables and extracting them out of an object or array

Question 22

What are higher order functions?

Answer 22

Methods or functions that accept other functions

Can be passed around

Can be returned without being invoked

Allows you to create extensible code

Question 23

What does single-threaded mean?

Answer 23

Process and execute only one task at a time

JavaScript is a single-threaded language

Question 24

What is synchronous programming?

Answer 24

Two or more things cannot be executed at the same time

Only one thing can be run to completion at any given time

Question 25

What is asynchronous programming?

Answer 25

Allows the computer to perform other tasks while waiting for an event to occur

Approaches to asynchronous programming:

• Callback functions
• Promises
• Async/await

Question 26

What is the call stack?

Answer 26

Mechanism in the JavaScript engine that keeps track of the order of function calls

Question 27

What is the callback queue?

Answer 27

Holding area for all the callbacks that are waiting to be put on the call stack and eventually executed

Question 28

What is the event loop?

Answer 28

Monitor the call stack and callback queue

Anytime the call stack is empty, the event loop takes the first task from the callback queue and pushes it onto the call stack, and runs it

Question 29

What are web APIs?

Answer 29

Process async operations while keeping the call stack clear of blocking operations

When a web API is done processing an async task, it’s not immediately executed on the call stack but pushed onto the callback queue

ie. setTimeout(), XMLHttpRequest(XHR), Fetch API, DOM event API