JS.Node Flashcards

Question 1

Q

What is a common error made when storing data as JSON ?

Answer

A

stringifying the data more than once before storage or not stringifying it at all.

Question 2

Q

what does array.filter() do?

Answer

A

filters an existing array and returns a new array that contains only the filtered values.

Question 3

Q

what does array.foreach() do?

Answer

A

Much like a for loop, it loops through each item in an array and performs a specified opperation on each item.

Question 4

Q

What does array.map() do?

Answer

A

Performs an opperation on each value within an array and returns a new array that contains the new mapped values.

Question 5

Q

What does array.reduce() do?

Answer

A

reduces the values in a given array to a single value.

Question 6

Q

What does it mean when we say that node uses “non-blocking I/O”?

Answer

A

It means that Node is asynchronous.

Question 7

Q

What are the two most common JSON commands?

Answer

A

JSON.stringify() and JSON.parse()

Question 8

Q

What does JSON.stringify do?

Answer

A

turns an object into a JSON object by adding double quotes to the keys and values in objects.

Question 9

Q

What does JSON.parse() do?

Answer

A

turns JSON objects into objects by removing the double quotes from the keys in an object.

Question 10

Q

Which module is used to work with files in node?

Question 11

Q

Which module is used to provide colourful feedback in the console?

Question 12

Q

How does the V8 engine works

Answer

A

Node itself is written in C++ and the V8 engine is embedded into it. When writing javascript. The javascript and it’s c++ bindings are passed into the V8 engine which converts it to machine language which can be run on a machine like a computer or a server.

Question 13

Q

The javascript engine used by Chrome

Question 14

Q

The javascript engine used by Edge

Question 15

Q

The javascript engine used by Firefox

Answer

A

Spidermonkey

Question 16

Q

The javascript engine used by Edge

Answer

A

Javascriptcore

Question 17

Q

What is a javascript engine?

Answer

A

A program that compiles javascript down to machine language

Question 18

Q

What is the difference between an AOT and JIT compiler?

Answer

A

AOT (ahead of time) compilers, will compile the entire program into machine code ahead of time.

JIT (Just in time) compilers used a combination of interpretation and compilation and will compile blocks of code just before it is neede.

JIT is slower than AOT but it makes it easier to debug and maintain code.

Question 19

Q

The V8 engine is multi-threaded. What are the 4 main threads and what are there functions?

Answer

A

Main thread: Fetch - compile -execute
Compiler thread: Codegen(ignition): converts to
machine code Crankshaft(turbofan): optimizes hot
code
Profiler thread: Monitors code and finds “hot” sections
Garbage collection thread: Garbage collection

Question 20

Q

How does crankshaft optimize code?

Answer

A

1. inlining: All the function in the "hot" path are replaced 
   by the actual code from the function to improve 
   performance
2. Hidden classes: The compiler generates hidden 
    classes that maintains the pointers to every object 
    property in a class structure so that it need not be 
    looked up everytime
3. inline caching: A cache of the object type that is most 
    likely to be passed to a function is maintained so that 
    unnecessary lookups are avoided

Question 21

Q

what is a “method”?

Answer

A

Methods are functions defined as a propertie of an object or class. methods are executed using the dot notation.

Question 22

Q

Why are arrow functions not well suited for use as methods in an object?

Answer

A

Because arrow functions do not bind their own “this” value, instead they access the “this” value of the context they were created in. This can make it difficult to access properties within objects. Instead it would be better to use an anonymous function which binds its own “this” value

Question 23

Q

What are arrow functions best used for?

Answer

A

callback functions

Question 24

Q

What should we use in the place of array.filter() when looking for a single item in a large array? Why?

Answer

A

Array.find()

Array.filter() will look through each item in a list one at a time. It will continue to do so even if the item we are looking for has been found.

Example. In a list of 1000 items, if the item we are looking for is at index 89, it will continue to look through the remaining 911 items before stopping.

Array.find() will stop after it finds the first instance of the desired item.

Question 25

Q

Array.find()

Answer

A

Looks for item in an array an returns the item (not new array) when found.

Returns “undefined” when nothing is found

Question 26

Q

How can I use chrome as a debugging tool?

Answer

A

add debugger to script
run “node inspect app.js commands” in terminal
navigate to chrome://inspect in the chrome browser
add folders to project
press esc to toggle console
press run top right hand corner
look at values in the right hand pane.

Question 27

Q

What are the 6 catagories of errors in javascript?

Answer

A

EvalError: Raised when the eval() functions is used in an incorrect manner.

RangeError: Raised when a numeric variable exceeds its allowed range.

ReferenceError: Raised when an invalid reference is used. Like when you use a variable that has not been declared

SyntaxError: Raised when a syntax error occurs while parsing JavaScript code.

TypeError: Raised when the type of a variable is not as expected. Like when you call .toUpperCase() on an integer

URIError: Raised when the encodeURI() or decodeURI() functions are used in an incorrect manner.

Question 28

Q

Javascript generates an error object when an error occurs. Which two propperties does it contain?

Answer

A

name

message

Question 29

Q

What is a stack trace?

Answer

A

A stack trace is a list of the functions, in order, that lead to a given point in a software program.A stack trace is essentially a breadcrumb trail for your software.It helps us understand the steps that lead to the error.

Question 30

Q

What is the biggest problem with anonymous functions?

Answer

A

They make it difficult to debug as they show up in the stack trace as anonymous.

Question 31

Q

console.trace()

Answer

A

A command that can be placed inside a function which then generates a stack trace for that funciton in the console or terminal.

Question 32

Q

Format of a stack trace

Answer

A

Error name
    function name, file, line number, column number

Question 33

Q

Which async function can be used to time the execution of a function?

Answer

A

setTimeout(callback, time in milliseconds)

Question 34

Q

Call stack

Answer

A

a call stack is a stack data structure which is based on the LIFO or last in first out principle. It uses four opperations namely push (to add data), pop (to remove data), peek (to return the value of the top item), isEmpty (returns true if stack is empty). Stacks emulate real life stacks like a stack of books or a can of tennis balls.

Whe a script runs, it wraps all of the code in the main() function. The main function gets pusched onto the call stack and executes, then any function called by main gets pushed onto the call stack on top of main. when the function has finished running it is poped of the call stack. and right at the end of the script the main funciton is popped of the stack.

Question 35

Q

Callback que

Answer

A

a que used by node to keep track of callback functions that are waiting to run.

Question 36

Q

Event loop

Answer

A

Adds functions waiting in the call back cue to the call stack when the stack is empty.

Question 37

Q

Explain what happens in the background when running asyncronous scripts?

Answer

A

Function gets pushed onto the callstack, the async event gets registered with the node api, when it is ready or complete it is moved to the callback cue where it waits for the event loop to add it to the call stack as soon as the call stack is empty. Once it is in the call stack it get executed.

Question 38

Q

How to initialize a new project as an NPM project?

Answer

A

run “npm init -y” in the root of the folder

Question 39

Q

package.json file

Answer

A

holds the metadata and information about dependencies and project configuration. It is generated automatically as soon as a project is initialized using npm init

Question 40

Q

Which npm module can be used ot make http requests?

Question 41

Q

simplest way to make an http request using request?

Answer

A

request({url:url,json:true},(error,response)=>{
console.log(response)
});

Question 42

Q

Which chrome extension presents json data in a more readable format?

Answer

A

JSON formatter

Question 43

Q

What are query strings?

Answer

A

a query string is the part of a uniform resource locator (URL) which assigns values to specified parameters. The query string commonly includes fields added to a base URL

query string format base_url?key=value&key=value

Question 44

Q

Which api provides geocoding or location based information?

Question 45

Q

How can errors be handled when using the npm request module to make http requests?

Answer

A

if(error){
    console.log("cannot connect)
}else if(response.body.error){
    console.log("cannot find info")
}else{
    console.log(response.body)
}

Question 46

Q

What is defensive programming?

Answer

A

Anticipating possible errors and pre-emptively writing code that solves the anticipated problem

A common example is the try/catch statements

Question 47

Q

What is a call back function?

Answer

A

A callback is a function that is to be executed after another function has finished executing

Functions that do this are called higher-order functions.

Any function that is passed as an argument is called a callback function.

Question 48

Q

What is a higher order function?

Answer

A

Any function that calls another function or that takes another funciton as an argument

Question 49

Q

What is a common convention used for callback functions?

Answer

A

someFunction(arg,(error,data)=>{

})

Question 50

Q

which function helps to parse user input into a valid URI component?

Answer

A

encodeURIComponent()

Question 51

Q

In which folder should code be placed that make api calls or HTTP requests?

Question 52

Q

What is callback chaining?

Answer

A

chaining together a number of functions via callbacks while passing arguments (data) between them.

Question 53

Q

What is callback hell?

Answer

A

When so many functions have been chained together that it becomes unreadable and difficult to understand.

Question 54

Q

What is object destructuring?

Answer

A

A method that extracts values from an object

const book = {
label:"To the moon and back",
price:56,
author:"Ryan holiday"
}

const {label, price} = book

Question 55

Q

How to rename a property during object destructuring?

Answer

A

currentName:newName

const book = {
label:"To the moon and back",
price:56,
author:"Ryan holiday"
}

const {label:title, price:cost} = book

Question 56

Q

How to set defualt values for properties when they don’t exist in an object during destructuring?

Answer

A

Using the assignment operator

const book = {
label:"To the moon and back",
price:56,
author:"Ryan holiday"
}

const {title, cost = 100} = book

Question 57

Q

How to destructure an object passed in as an argument into a function?

Answer

A

by using curly braces.

const book = {
label:"To the moon and back",
price:56,
author:"Ryan holiday"
}

function inventory(type, {label, price, author}){
    some code here
}

Question 58

Q

Which core modules are used ot make http requests

Answer

A

http and https

Question 59

Q

How to make an HTTP request using the https core module?

Answer

A

const https = require(‘https’);

const request = https.request(url,(response)=>{
    let data = '';

    //event listener that adds the chunks of data returned 
      from the server. Data is returned as a series of buffers 
      which needs to be converted to a string
    response.on('data', (chunk)=>{
        data =  data + chunk.toString()

});

    //runs when response is complete converting the 
      string to an object
    response.on('end',(chunk)=>{
        const body = JSON.parse(data);
        console.log(body)

})

});

//Event listener that handles errors
request.on('error', (error)=>{
    console.log('An error ', error)
});

//This code snippet tells the request to run, it won't run without it. 
request.end();

Question 60

Q

Which npm module is commonly used to create servers?

Question 61

Q

directory structure for node projects

Answer

A

lib/ is intended for code that can run as-is

src/ is intended for code that needs to be manipulated before it can be used

build/ is for any scripts or tooling needed to build your project

dist/ is for compiled modules that can be used with other systems.

bin/ is for any executable scripts, or compiled binaries used with, or built from your module.

test/ is for all of your project/module’s test scripts
unit/ is a sub-directory for unit tests

integration/ is a sub-directory for integration tests
env/ is for any environment that’s needed for testing

Question 62

Q

What is the src directory intended for?

Answer

A

is intended for code that needs to be manipulated before it can be used

Question 63

Q

What is the most basic server setup using express?

Answer

A

const express = require(‘express’); //import express

const server = express(); //create a server

server.get(‘’,(req,res)=>{ //handle request for homepage
res.send(‘Hello world’) //sends a response
});

const port = 3000; //sets the port to listen on

server.listen(port,()=>{ //starts the server
console.log(‘Server is up and running on port ‘ + port)
});

Question 64

Q

What is the simplest way to handle a request to the express server?

Answer

A

//by setting up a route

server.get(‘’,(req,res)=>{
res.send(‘Hello world!’)
})

Answer 57

A

__dirname

__filename

Answer 58

A

To serve up static pages such as html, css and javascript

Answer 59

A

const publicDirectoryPath = path.join(__dirname, ‘../public’)

server.use(express.static(publicDirectoryPath))

Answer 60

A

inside the css folder within the public directory

Answer 61

A

within the public directory

Answer 62

A

inside the js folder within the public directory

Answer 63

A

in the img folder within the public folder

Answer 64

A

hbs and handlebars

Answer 65

A

A template engine enables you to generate dynamic content in express. At runtime, the template engine replaces variables in a template file with actual values, and transforms the template into an HTML file sent to the client. This approach makes it easier to design an HTML page.

Answer 66

A

//install handlebars module
npm i hbs

//set up view engine
server.set('view engine','hbs')

//create directory for views in the root directory

Answer 67

A

//using a route 
//use res.render() instead of res.send()
// render takes two arguments 
// the file to be rendered's name
// an object with the dynamic content.

server.get(‘’,(req,res)=>{
res.render(‘filename’, {property:value})
});

Answer 68

A

const viewsDirectoryPath = path.join(\_\_dirname,"../templates");
app.set('views',viewsDirectoryPath);

Answer 69

A

root => templates => views, partials

server.set(‘views’, path)

Answer 70

A

hbs.registerPartials(partialsDirectoryPath);

Answer 71

A

Answer 72

A

The wild card *

Answer 73

A

Node looks through each route handler in the order they are written. Node will ignore all route handlers after the wild card.

Answer 74

A

import modules
create server
set public directory
create template machine
set views directory
register and set partials directory
Set up route handlers
Set up 404 route handler
Start server

Answer 75

A

req.query

Answer 76

A

if(!req.query.searchterm){
return res.send({
error:”You must provide a search term”
})

Answer 77

A

fetch(url).then((response)=>{
      response.json().then((data)=>{
           if(data.error){
              console.log(data.error)
           }else{
              console.log(data)
          }
       })
})

Answer 78

A

//save the form element
const weatherForm = document.querySelector('form');

//Save the input element
const search = document.querySelector("input")

//Add an event listener
weatherForm.addEventListener('submit',(e)=>{
    e.preventDefault() //prevent default
    const location = search.value() //get the value
    console.log('Testing form')
});

Answer 79

A

//use e.preventDefualt() to prevent default behaviour
weatherForm.addEventListener(‘submit’,(e)=>{
e.preventDefault()
console.log(‘Testing form’)

Answer 80

A

Because most browsers or websites will block cross-origen requests due to the specter vulnerabillty that would allow one access to a websites files and and content.

Answer 81

A

node_modules

Answer 82

A

npm install

Answer 83

A

Add a .gitignore file to the root directory and write node_modules/ in the file

Answer 84

A

npm run scriptname

Answer 85

A

Because, when other developers download the project onto their machine, the global modules are not included, which means that the program wont run as expected or at all.

Answer 86

A

npm uninstall -g moduleName

Answer 87

A

npm i module –save-dev installs a package as a dev dependency

Answer 88

A

dev dependencies are not intalled in a production environment, only in a development environment

Answer 89

A

No. The command only works when it is installed globally. In order to run it locally, you have to create a dev script

Answer 90

A

Representational State Transfer

Answer 91

A

Create: POST
Read: GET
Update: PATCH
Delete: DELETE

Answer 92

A

Request line: Method, Path, protocol
Multiple Request headers: Key/Value pairs
a. ) Accept: application/json -tells the api what kind of data to return
b. ) Connection: Keep-Alive -tells the api to keep the connection open as more requests are likely to be made.
c. )Authorization: Bearer ey34Jdetc. -contain authorization keys
Request body that contains Data provded as JSON

Answer 93

A

Status line: Protocol, status code, text representation of status code
Response Headers: Date, server, content type
Body: contains JSON data

Answer 94

A

nodemon
express
request
mongodb
mongoose
validator
hbs
bcryptjs
jsonwebtoken
multer
sharp
@sendgrid/mail
env-cmd
jest
supertest

Answer 95

A

server.use(express.json());

this will make json available as an object accessibel on req.body

Answer 96

A

100: informational
200: success
300: redirection
400: Client Error codes
500: Server error codes

Answer 97

A

httpstatuses.com

100: informational
200: success
300: redirection
400: Client Error codes
500: Server error codes

Answer 98

A

import the mongoose client

require(‘./db/mongoose.js’)

2. import the models
const User = require('./db/models/user.js')

instantiate an instance of the model in to be used by the routes

Answer 99

A

//Post request creating a new user

server.post(“/users”,(req,res)=>{

const user = new User(req.body);

user.save().then(()=>{
    res.send(user);
}).catch((error)=>{
    res.status(400).send(error)
}) });

Answer 100

A

//in this example the Task model is not instantiate, instead .find() is called directly on the model itself

server.get("/tasks",(req,res)=>{
    Task.find({}).then((tasks)=>{
        res.send(tasks)
    }).catch((error)=>{
        res.status(500).send(error)
    })
});

//in retrieving a specific document, an id is added to the end of the url using :id, which is then accessed using req.params.id.

server.get("/tasks/:id",(req,res)=>{
    const id = req.params.id;
    Task.findById(id).then((task)=>{
        if(!task){
            return res.status(404).send()
        }
        res.send(task);
    }).catch((error)=>{
        res.status(500).send(error)
    })
});

Answer 101

A

An important difference with .every() is that the test function may not always be called for every element in the array. Once the testing function returns false for any element, no more array elements are iterated. Therefore, the testing function should usually have no side effects.

.forEach() returns nothing - It iterates the Array performing a given action for each item in the Array.

Answer 102

A

const password = 'Red12345!'
const hashedPassword = await bcrypt.hash(password,8);

Answer 103

A

No, it is one way

Answer 104

A

const isMatch = await bcrypt.compare(userPassword, storedHashPassword)

Answer 105

A

Allows for the customization of mongoose models

Answer 106

A

.findById

.update

Answer 107

A

jasonwebtoken

Answer 108

A

it has 3 parts: Header, Body,

The Header is a base 64 encoded json string that contains meta information about the type of token and the algorithem used ot generate it.

Answer 109

A

it has 3 parts: Header, Body, signiture

The Header is a base 64 encoded json string that contains meta information about the type of token and the algorithem used ot generate it.

The Body is a base 64 encoded string that contains the payload and a timestamp

The signitureis a base 64 encoded string that contains the signiture used to verify the token

Answer 110

A

const token = jwt.sign({_id:’abc123’}, ‘secretKey’);

Answer 111

A

const data = jwt.verify(token,’secretKey’);

returns a payload object if verified, otherwise it throws an error

Answer 112

A

userSchema.statics.methodName(){

}

Answer 113

A

userSchema.methods.methodName(){}

Answer 114

A

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.

Middleware functions can perform the following tasks:

Execute any code.
Make changes to the request and the response objects.
End the request-response cycle.
Call the next middleware function in the stack.
If the current middleware function does not end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.

Answer 115

A

//middleware needs to be created above the route handlers for it to work

server.use((req,res,next)=>{
    if(req.method === 'GET'){
        res.send('GET requests are disabled')
    } else {
        next()
    }
});

Answer 116

A

placed in the header

Key: Authorization
Value: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJfaWQiOiI1ZTk5NjI0M2FiZTA3ZTBkNDAzYjE5YzUiLCJpYXQiOjE1ODcxNzk5OTN9.eGfK8IoVIbbpyH2SxTR9_Liw7YTGDisVbc5MDMdaqFE

Answer 117

A

//test
const Task = require('./db/models/task');
const User = require('./db/models/user');

const main = async ()=>{
const user = await User.findById(‘5e99a4a5e97c45531011fe95’);
await user.populate(‘tasks’).execPopulate();
console.log(user.tasks)
}

main();

Answer 118

A

when defining the modelSchema pass the following object in as the second argument

{timestamps:true}

Answer 119

A

const upload = multer({
    dest:"avatar",
    limits:{
        fileSize:1000000
    },
    fileFilter(req,file,cb){
        if(!file.originalname.endsWith('.pdf')){
            cb(new Error('Please upload a pdf!'))
        }

    cb(undefined,true)
} });

router.post(‘/users/uploads/avatar’,upload.single(‘avatar’), (req,res)=>{
res.send()
});

Answer 120

A

const upload = multer({
    dest:"avatar",
    limits:{
        fileSize:1000000
    },
    fileFilter(req,file,cb){
        if(!file.originalname.match(/\.(jpg|jpeg|png)$/)){
            return cb(new Error('Please upload a word document!'))
        }

    cb(undefined,true)
} });

Answer 121

A

router.post(‘/users/uploads/avatar’,upload.single(‘avatar’), (req,res)=>{
res.send()
},(error,req,res,next)=>{
res.status(400).send({error:error.message})
});

Answer 122

A

Answer 123

A

Need to set the header

res.set(‘content-type’,’image/jpg’)

or

res.set(‘content-type’,’application/json’)

Answer 124

A

const buffer = await sharp(req.file.buffer).resize({width:250, height:250}).png().toBuffer();
        req.user.avatar = buffer;

Answer 125

A

install npm i env-cmd
Create a dev.env file
Setup variables in the file
access variables in relevant locations using process.env.ENV_NAME
Set the dev script to “env-cmd -f ./dev.env nodemone src/index.js”

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JS.Node Flashcards

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