JavaScript

Question 1

What is JavaScript?

Answer 1

A scripting language for creating dynamic web page content.

Question 2

ecmascript

Answer 2

A standard on how a language should work (ex. javascript) and what features it should have.

Question 3

Destructuring

Answer 3

a faster way to access data from arrays, objects, and maps by setting each value as new variables.

Question 4

map (array method)

Answer 4

map creates a new array by calling a function on every array element. It does not change the original array.

Question 5

spread operator (ES6) (…)

Answer 5

allows you to quickly copy all or part of an existing array or object into another array or object using “…”

Question 6

rest operator (…)

Answer 6

allows a function to take an indefinite amount of arguments and bundle them in an array. The rest parameter must be placed at the end of the arguments to gather all remaining arguments.

Question 7

preventDefault (method)

Answer 7

method that tells the form we’ll handle what happens when a form is submitted. By default, when a form is submitted, the information is sent through a URL.

Question 8

named export vs default export

Answer 8

named export must use the same name. For default, you can come up with any name. You can only have one default export per file. example for named export: “export const books …”. example for default export: “export default books”.

Question 9

named import vs default import

Answer 9

named import must be the same name. for default export, we can come up with whatever we want. example- for named import: “import {books} from “./books.js””. example for default import “import banana from “./books.js””.

Question 10

synchronous

Answer 10

Tasks are completed one after another. JavaScript is synchronous and single-threaded.

Question 11

asynchronous

Answer 11

Tasks are completed parallel to one another. This is a lot better performance-wise than synchronous. In JS, async is provided by Events. Async patterns include - callbacks, promises, and async/await.

Question 12

Blocking

Answer 12

Blocks the execution. This is synchronous.

Question 13

Non-blocking

Answer 13

Does not block the execution of further-operations. This is mainly asynchronous. Non-blocking is more performant than blocking code.

Question 14

event

Answer 14

An event is JS interaction with HTML elements. Such as when the page loads, or when a button is clicked.

Question 15

event loop

Answer 15

A constantly running process that coordinates the tasks between the call stack and callback queue to achieve concurrency.

Question 16

callback queue

Answer 16

A queue that keeps all callback functions ready for execution and executes after the call stack is empty. This operates on the FIFO principle. The callback functions are pushed to the call stack to execute when the event loop finds an empty call stack.

Question 17

callback

Answer 17

a function passed as an argument to another function.

Question 18

promise

Answer 18

an object that represents the eventual success or failure of an asynchronous task and its resulting value.

Question 19

.then() promise method

Answer 19

When the promise is resolved/fulfilled, you can access the data and display the data within this method. It immediately returns an equivalent Promise object, allowing you to chain calls to other promise methods- creating a promise chain.

Question 20

.catch() promise method

Answer 20

When a promise is rejected/fails, you can access the error information and display the error information in this method. It immediately returns an equivalent Promise object, allowing you to chain calls to other promise methods- creating a promise chain.

Question 21

.finally() promise method

Answer 21

The last stage of the promise is where the promise has “settled”. This is useful when you want to do something when the promise has settled.

Question 22

.all() promise method

Answer 22

This takes multiple promises as input and returns a single Promise. The returned promise is fulfilled if all promises are able to be resolved. It rejects when any of the input’s promises rejects, returning the first reject. This runs the inputs in parallel so it is performant.

Question 23

fetch() method

Answer 23

Used to request data from a server. The request can be any type of API that returns the data in JSON or XML. It requires one parameter, the URL to request, and returns a promise.

fetch(URL) // api for the get request
.then(response => response.json())
.then(data => console.log(data));

Question 24

async/await

Answer 24

async/await allows you to write asynchronous code using the async and await keywords. The async keyword is used to declare a function as asynchronous, while the await keyword is used to pause the execution of a function until a promise is resolved. Async returns a promise.

When setting up async functions, it is good practice to setup try…catch within the function to catch any errors.

Question 25

for (VARIABLE of ITERABLE) {}

Answer 25

a statement that loops through the values of an iterable object such as arrays, strings, maps, nodelists, etc.

Question 26

for (KEY in OBJECT) {}

Answer 26

a statement that loops through the properties of an object.

Question 27

fetch API

Answer 27

Starts the process of fetching a resource from a server. It returns a Promise that resolves to a Response object. This is a modern promise-based replacement for XMLHttpRequest.

Question 28

short-circuit evaluation

Answer 28

For Logical Operators (&&, ||). In JavaScript, an expression is evaluated from left to right until there is a confirmed result. If the condition is met before everything is evaluated, the expression will short-circuit and return the result.

Example for &&: “false && true” will return false and the true is never evaluated.

Example for ||: “true || false” will return true and false is never evaluated.

Question 29

closure

Answer 29

an inner function always has access to the variables and parameters of the outer function, even after the outer function has returned.

Question 30

When using methods to manipulate data.

Answer 30

When using string, array, object, document, etc. methods, you are using methods available on the String, Array, Object, or Document object. When you create a string for example, that string is automatically created as an instance of String (object), allowing you to use the common methods and properties available on it.

Question 31

class

Answer 31

a template for creating multiple objects with the same list of properties but different data values saved in it. A class on its own doesn’t do anything, so every instance of the class that is created is performed by the constructor. We pass values to the constructor for any internal state we want to initialize in the new instance. There are no classes in JavaScript, there is only Object. To be more precise, JavaScript is a prototype-based Object Oriented Language, which means it doesn’t have classes, rather it defines behaviors using a constructor function and then reuses it using the prototype.

Question 32

constructor

Answer 32

A special function that creates and initializes an object instance of a class. The purpose of a constructor is to create a new object and set values for any existing object properties. To call a constructor, you use the “new” keyword.

Question 33

prototype

Answer 33

a mechanism in which javascript objects inherit features from one another. Every object in JS has the built-in hidden property called “prototype”. Prototype is an object itself and will have it’s own prototype, making a “prototype chain”.

Question 34

OOP

Answer 34

A design to model a system as a collection of objects, where each object represents some particular aspect of the system. The four principles of OOP are Abstraction, Encapsulation, Inheritance, and Polymorphism.

Question 35

Abstraction (OOP)

Answer 35

show only the essential information and hide the implementation details. You cannot create an instance of an Abstract Class.

Question 36

Encapsulation (OOP)

Answer 36

Grouping data and functions into one component and limiting access and functionality to the component to keep it secure.

Question 37

Inheritance (OOP)

Answer 37

A class or object inherits the attributes and methods of another class or object.

Question 38

Polymorphism (OOP)

Answer 38

The ability for a variable, function, or object to take on multiple forms.

Question 39

super (keyword)

Answer 39

is used to call the constructor of its parent class to access the parent’s properties and methods.

Question 40

extends (keyword)

Answer 40

Used to create a child class of another class (parent).

Question 41

function declaration vs function expression

Answer 41

Function declarations are functions with a name. This will add the function to the global scope; so you can call it throughout your code. A function expression is a function without a name, making it an “anonymous function”. This is useful for limiting where the function is available keeping your global scope light. https://www.freecodecamp.org/news/when-to-use-a-function-declarations-vs-a-function-expression-70f15152a0a0/

Question 42

filter() method

Answer 42

Creates a new array filled with elements that pass a test provided by a function.

Question 43

switch statement

Answer 43

This is used to select one of many code blocks to be executed.
Format:
switch(EXPRESSION) {
case xxxxx:
// code block
break;
case yyyyy:
// code block
break;
default:
// code block. The default keyword specifies code to run if there is no case match.
}

Question 44

“return” statement in a function

Answer 44

This means “the function is done running, here is the result you can show to the parent context. No need to read further.”

Question 45

truthy vs falsy values

Answer 45

Falsy - false, 0 (zero), “” (empty string), null, undefined, NaN.
Truthy - All other values, including objects and arrays.

Question 46

! (operator)

Answer 46

The ! operator is a logical operator that negates a boolean value. It is equivalent to the not operator in other programming languages.

Example:
if(!false) {
console.log(true);
}

Question 47

Set (object)

Answer 47

A collection of unique values.
Example:
new Set([“a”, “a”, “b”, “c”])
//output: Set(3){‘a’, ‘b’, ‘c’}

Question 48

FormData API

Answer 48

provides a way to construct a set of key/value pairs representing form fields and their values, which can be sent using the fetch() or XMLHttpRequest.send() method.