JavaScript Cert Flashcards
Two main properties of an Error object thats passed as an arguement to catch in a try..catch?
Name
Message
Advantages of using Node.js ? (name three)
Asynchronous
Open Source
Non-Blocking
Valid Events when interacting with the browser?
-mousedown
-contextmenu
-keyup
describe “Modules”
-Modules can load each other
-special directives such as export and import can be used interchangeably
-a module is just a file that may contain a class or a library of functions for a specific purpose
correct syntax for accessing the property of an object in JavaScript?
The correct syntax for accessing the property of an object is:
- objectName.property
- objectName[“property”]
- objectName[expression]
name some “JavaScript Frameworks”
Angular
Node.js
React
“Falsy” values:
- 0
- null
- undefined
- NaN
- ”” or ‘’ (empty string)
- false
valid data types in JavaScript:
-null
-undefined
-Object
-Symbol
-Number
-String
-BigInt
-Boolean
correct way to look for an element by its ID
document.getElementById(‘element’);
What are the 2 most important items in your npm package.json file if you plan to publish a package?
-Name
-Version
*these 2 together establish a unique identifier for your published package
*optional if not going to publish
3 Phases of Event Propagation in standard DOM
The standard DOM Events describes 3 phases of event propagation:
Capturing phase – the event goes down to the element. (value=1)
Target phase – the event reached the target element. (value=2)
Bubbling phase – the event bubbles up from the element. (value=3)
*there is also a None phase before capturing phase (not technically event propagation though) (value=0)
how do Events in the Target Phase trigger?
will trigger all listeners on an element in the order they were registered. Hence, the outer message will be displayed first, then the inner message.
What is a “Curried Function”?
“Curried Function”: Currying is a transform that makes f(a, b, c) callable as f(a)(b)(c). The advantage of currying is that it allows us to easily get the partials. For example, we might just want to call f(a) or f(a)(b), depending on our own needs.
If a function does not return a value, its value is equal to = __________
undefined
should you ever add a “new line” between return and the value? (Y/N)
No
jQuery libarary defines a function with ______?
Lodash library defines its main function with ______?
jQuery = “$”
Lodash = “_”
3 most popular “Testing Frameworks” used by JavaScript developers?
-Puppeteer
-Mocha
-Jasmine
Describe “Mocha” Testing Framework?
-feature rich JavaScript test framework running on Node.Js and in the browser
-makes Asynchronous testing simple and fun (synchronous testing also)
Describe “Puppeteer” Testing Framework?
Node library which provides a high-level API to control headless Chrome or Chromium over the DevTools Protocol
Describe “Jasmine” Testing Framework?
Behavior Driven development framework,
for testing JavaScript code
what is JUnit ?
unit testing framework for the** Java** (not JavaScript) programming language
describe a Callback function ?
a function passed into a function to be called later
by default, the sort() function sorts values as _____?
strings
How would we sort an array in ascending order?
arr.sort((a,b) => a - b );
How would we sort an array in descending order?
arr.sort((a,b) => b - a);
what are the different states for a promise?
-Pending : initial state, neither fulfilled or rejected
-Fulfilled : operation completed sucessfully
-Rejected : operation failed
3 ways to define a Function
-Declaration decl() *inherits current scope
-Expression expr() *inherits current scope
-Constructor cons() *inherits global scope
What is the nature of how Events happen in the DOM?
When an event happens on a element, it first runs the handlers on it, then on its parent, then all the way on up to other higher ancestors
–> ie Events Bubble up the event chain in DOM from innner elements to outer elements (children to parents)
the standard DOM Events describes 3 phases of event propagation:
- Capturing phase - the event goes down to the element
- Target phase - the event reached the target element
- Bubbling phase - the event bubbles up from the element
What is DOM?
DOM - Document Object Model
–> a programming interface for HTML and XML documents
–>represents the page so that programs can change the document structure, style, and content
–>represents the page as nodes and objects (so languages like JavaScript can connect to page)
What is JavaScript Prototypical Inheritance?
-an in-memory object that defines properties and functions of other objects
*despite not having classes as defined by classical languages, JavaScript still has an inheritance model which is called Prototype Inheritance
JSON characteristics?
-data-interchange format
-JSON spec is based on object literal notation
Object Literal Notation characteristics?
-programming syntax
What do you call errors that happen in valid code?
runtime errors
(or) exceptions
What do you call errors that happen in invalid code (code that can’t be understood by the engine because it is syntactically wrong)?
parse-time errors
function types related to Promises ?
*think method handling architecture design
-then
-catch
-finally
A developer wants to use a module called DatePrettyPrint. This module exports one default function called printDate(). How can a developer import and use the printDate() function?
import printDate from ‘/path/DatePrettyPrint.js’;
printDate();
-simplest way to do it,
notice parenthesis when naming export function are not needed
-notice you can just call function after imported, instead of using dot notation to call from its class
how would you import all methods from a module called DatePrettyPrint
import * from ‘/path/DatePrettyPrint.js’;
note: think –> import *
what is correct way of creating a new instance of a object?
say example function is:
function Car(size,model){
this.size =size;
this.model=model;
}
new Car(‘large’,’Audi’);
note: New is the keyword
characteristics involving classes in JavaScript?
-there are two ways to define a class –> class declaration (or) class expression
-class expressions can be named or unnamed and the name given to a named class expression is local to the class’s body
-classes are the “special functions” which are primarily syntactical sugar over JavaScript existing prototype-based inheritance
splice() syntax
array.splice(start[, deleteCount[, item1[, item2[, …]]]])
For example:
arr.splice(1,3,4,5) means that at index 1, remove 3 items, then add items 4 and 5 into that index. With an array of [1,2,3], first it removes 3 items from index 1 (in this case, only 2 items because it reaches the end of array) . The array becomes [1] and then it adds the other 2 items to the array, which the array becomes [1,4,5].
describe the event DOMContentLoaded
event should be specified in the event listener as the DOMContentLoaded event will be fired when the DOM content is loaded, without waiting for images and stylesheets to finish loading.
document.addEventListener(“DOMContentLoaded”, function(){
// specify your code here
});
describe the event load
event is fired when the whole page has loaded, including all dependent resources such as stylesheets and images.
document.addEventListener(“load”, function(){
// specify your code here
});
3 main White Box testing techniques ?
-Path coverage
-Branch coverage
-Statement coverage
White Box testing vs Black Box testing ?
White Box: test the actual code, see and test the internal code
Black Box: test software from user’s point of view, perform testing without seeing the internal system code
Valid Cookie options:
path=/
domain=domain.com
expires= 19 Jan 2038 03:14:07 GMT (*gmt string)
max-age=3600
secure
samesite
when using functions as event handlers, which parameters can they accept?
only events
-only events can be passed into an event handler function
when using functions as event handlers, which parameters can they accept?
only events
-only events can be passed into an event handler function
-called Event Object, it is automatically passed to event handlers to provide extra features and information
-the target property of the event object is always a reference to the element that the event has just occurred upon
what does the Object.assign() method do
-copies all enumerable own properties from one or more source objects to a target object
–> basically copies the object, so it doesn’t reference the object in any way, so if a value changes in original object, your copied object does not change
what does the Object.entries() method do?
method returns an array of a given object’s own enumerable string-keyed property [key, value] pairs.
–>ex: let user = {
name: “John”,
age: 30
};
console.log( Object.entries(user) ); // what is the output?
answer: [ [“name”, “John”], [“age”, 30] ]
do arrow functions () => {} have access to this ?
no
-arrow functions are not bound to the object, so they do not have access to this
some main things to do to convert a non-async function with promise chaining to async function
-replace .then calls with await
-make the function async, ie put async keyword at beginning of function
*these are the main ideas
some main things to do to convert a non-async function with promise chaining to async function
-replace .then calls with await
-make the function async, ie put async keyword at beginning of function
*these are the main ideas
some main things to do to convert a non-async function with promise chaining to async function
-replace .then calls with await
-make the function async, ie put async keyword at beginning of function
*these are the main ideas
What is the use of breakpoint in JavaScript?
Breakpoints allow you to pause the code execution so that you can examine the current JavaScript values
2 ways to set the class for a DOM element
document.getElementById(“myId”).className = “myClass”;
document.getElementById(“myId”).setAttribute(“class”,”myClass”);
2 ways to set the class for a DOM element
document.getElementById(“myId”).className = “myClass”;
document.getElementById(“myId”).setAttribute(“class”,”myClass”);
facts about localStorage and sessionStorage
-both allow us to store key/value pairs in the browser
-both have their own storage limits (usually limited to 5mb(per domain))
-sessionStorage –> survives page refresh, but not browser restarts or tab closing/opening (exists only within the current browser tab)
-localStorage –> will not expire and it will remain after the browser restarts or even a OS reboot
valid ways of Initializing an array?
new Array()
Array()
[]
*remember that JavaScript is case-sensitive, so array() is not the same as Array() and will throw a ReferenceError message
queue data structure for array
define these queue array functions:
push()
shift()
unshift()
-order collection of elements
-FIFO
push() –> appends a element to end
shift() –> gets 1st element and removes it, advancing queue, so 2nd element advances to new 1st element
unshift() –> add element to the beginning
*push() and unshift() can add multiple elements at once
*push() and pop() run fast, shift() and unshift() run slow
stack data structure for array
define these stack array functions:
push()
pop()
-LIFO
push() –> add an element to the end
pop() –> takes an element from the end
What are valid ways of accessing the defined car’s properties?
const car = {
color: ‘blue’,
‘the color’: ‘the blue’
}
car.color
car[‘color’]
let theColor = ‘the color’; car[’${theColor}’]
Jest is a popular JavaScript testing framework, name some attributes/characteristics of it?
-maintained by Facebook
-highly preferred framework for applications based on React
-compatible with NodeJS, React, Angular, VueJS and other Babel based projects
-comes bundled with some interesting features like snapshot testing and a built-in tool for code coverage
-easy mocking
-free
-large community
Jest is a popular JavaScript testing framework, name some attributes/characteristics of it?
-maintained by Facebook
-highly preferred framework for applications based on React
-compatible with NodeJS, React, Angular, VueJS and other Babel based projects
-comes bundled with some interesting features like snapshot testing and a built-in tool for code coverage
-easy mocking
-free
-large community
when defining the package version, what are the rules for semantic versioning and its version numbers?
Tilde symbol ~ includes everything greater than a particular version in the same minor range.
–>~1.0.0 allows versions 1.0.0, 1.0.1, 1.0.2, and so on
Carat symbol ^ includes everything greater than a particular version in the same major range.
–>^1.0.0 allows versions 1.0.0, 1.0.1, 1.0.2, 1.1.0, 1.2.3 and so on.
Map methods:
also what is a Map?
new Map() – creates the map.
map.set(key, value) – stores the value by the key.
map.get(key)– returns the value by the key, undefined if key doesn’t exist in map.
map.has(key) – returns true if the key exists, false otherwise.
map.delete(key) – removes the element (the key/value pair) by the key.
map.clear() – removes everything from the map.
map.size – returns the current element count.
*Map –> collection of keyed data items (just like an object)
Set methods:
also what is a Set?
new Set([iterable]) – creates the set, and if an iterable object is provided (usually an array), copies values from it into the set.
set.add(value) – adds a value, returns the set itself.
set.delete(value) – removes the value, returns true if value existed at the moment of the call, otherwise false.
set.has(value) – returns true if the value exists in the set, otherwise false.
set.clear() – removes everything from the set.
set.size – is the elements count.
*Set –> special type collection - “set of values” (without keys)
Numeric Conversions:
for values like:
-undefined
-null
-true/false
-string
-error
Value Becomes…
undefined –> NaN
null –> 0
true / false –> 1 / 0
string –> 0…The string is read “as is”, whitespaces (includes spaces, tabs \t, newlines \n etc.) from both sides are ignored. An empty string becomes 0.
error –> NaN.
Event type ouput sequence when a user presses a key?
keydown
keypress
keyup
Event type output sequence when a user clicks the mouse?
mousedown
mouseup
click
Event type output sequence when a user clicks the mouse?
mousedown
mouseup
click
Event type output sequence when a user clicks the mouse?
mousedown
mouseup
click
event handler syntax?
element.addEventListener(event,handler,[options]);
3 ways to assign event handlers
HTML attribute: onclick=”…”.
DOM property: elem.onclick = function.
Methods: elem.addEventListener(event, handler[, phase]) to add, removeEventListener to remove.
are ** and math.pow() the same?
yes, ** raises the first operand to the power of the second operand
ex:
8 ** 2 = 64
2 **1 = 2
how do you create a BigInt data type?
appending n to the end of an integer or by calling the BigInt() function
result??
console.log( typeof 123n );
console.log( typeof 0b10 );
console.log( typeof 123e-1 );
console.log ( typeof +”123” );
console.log( typeof 123f );
console.log( typeof 123n ); // “bigint” (appended n at end makes it a BigInt)
console.log( typeof 0b10 ); // “number” (b is for base, so 0 base 10 is how its read)
console.log( typeof 123e-1 ); // “number” (e is for expontent))
console.log ( typeof +”123” ); // “number”
console.log( typeof 123f ); // SyntaxError: Invalid or unexpected token
which of the console API allows logging and sorting the data?
console.table(data);
how would you remove a package from your node_modules directory?
npm uninstall <package_name></package_name>
what happens to the sort() function if you do not pass in a function that specifies a sorting method
sorts values as “strings”
ex: let arr = [ 100, 1, 2, 15, 3 ];
gets sorted to –> [1,100,15,2,3]
if you define a function with the new keyword, than what lexical environment can that function access?
Global scope
so if you define a function within a function with the new keyword, it won’t be able to access variables local to the function it was created in, like seen below, new function can’t access “value” of outer function, so throws a Reference error
function getFunc(value = “test”) {
let func = new Function(‘console.log(value)’);
return func;
}
getFunc(“hello”)();
correct way of writing IIFE (Immediately Invoked Function Expression)?
(function () {
// write your js code here
})();
And here’s the arrow function style:
(() => {
// write your js code here
})() ;
What is a IIFE?
Immediately Invoked Function Expression
–> JavaScript function that runs as soon as it is defined, also known as a “Self Executing Anonymous Function”
valid ways to getting the current timestamp?
new Date().getTime()
Date.now()
2 ways to filter an array by a query string?
arr.filter( el => el.toLowerCase().includes(query) )
arr.filter( el => el.toLowerCase().indexOf(query.toLowerCase()) !== -1 )
what does Object.preventExtensions() do?
static method prevents new extensible properties from ever being added to an object
list of some common built-in modules of NodeJS
(*just review this card answer)
assert - provides a set of assertion functions for verifying invariants
buffer - represent a fixed-length sequence of bytes
child_process - provides the ability to spawn child processes
cluster - splits a single Node process into multiple processes
crypto - provides cryptographic functionality
dns - enables name resolution and DNS lookups
events - handles events
fs - provides an API for interacting with the file system
http - provides an implementation of the HTTP protocol
http2 - provides an implementation of the HTTP/2 protocol
https - provides an implementation of the HTTPS protocol
net - provides an asynchronous network API for creating stream-based TCP or IPC servers
os - provides operating system-related utility methods and properties
path - provides utilities for working with file and directory paths
querystring - provides utilities for parsing and formatting URL query strings
readline - provides an interface for reading data from a Readable stream one line at a time
stream - provides an API for implementing the stream interface
string_decoder - provides an API for decoding Buffer objects into strings
timers - exposes a global API for scheduling functions to be called at some future period of time
tls - provides an implementation of the Transport Layer Security (TLS) and Secure Socket Layer (SSL) protocols that is built on top of OpenSSL
tty - provides classes used by a text terminal
url - provides utilities for URL resolution and parsing
util - supports the needs of Node.js internal APIs
v8 - exposes APIs that are specific to the version of V8 built into the Node.js binary
vm - enables compiling and running code within V8 Virtual Machine contexts
zlib - provides compression functionality
describe:
–> call()
–> apply()
–> bind()
call() –> method calls a function with a given this value and arguments provided individually
apply() –> method calls a function with a given this value, and arguments provided as an array or an array-like object
bind() –> method calls a function with a given this value, and a given sequence of arguments individually
When we install a specific package, we usually run the command line npm install package. However, we might also see the command line with an optional parameter –save or –save-dev
describe what –save and –save-dev are:
–save installs and adds the entry to the package.json file dependencies
–save-dev installs and adds the entry to the package.json file devDependencies
what is JSON.stringify(value[,replacer[, space]]) ?
method returns a JSON string corresponding to the specified value, optionally including only certain properties or replacing property values in a user-defined manner
–>all instances of undefined are replaced with null
what happens to “Reject” in Promises?
caught by closest .Catch() block, like an error would be
what happens to “Reject” in Promises?
caught by closest .Catch() block, like an error would be
which client-side storage options can store data that is persistent across page reloads or even after the browser is closed? (name 3)
-Local Storage
-Cookies
-IndexDB
what does event.stopPropagation() do?
what does event.stopImmediatePropagation do?
what does event.preventDefault() do?
event.stopPropagation() –> method prevents any parent handlers from being executed. It prevents further propagation of the current event in the capturing and bubbling phases
event.stopImmediatePropagation –> method prevents any parent handlers as well as other handlers from being executed
event.preventDefault –>
code snippet for setting up MySQL connection in NodeJS
*note: just look at this one
var mysql = require(‘mysql’);
var con = mysql.createConnection({
host: “localhost”,
user: “yourusername”,
password: “yourpassword”
});
con.connect(function(err) {
if (err) throw err;
console.log(“Connected!”);
});
how to turn JSON data into stuff you can read in the console log and what not
JSON.Stringify();
*obviously value is inside the stringify() function parenthesis
what does JSON.parse() do?
static method parses a JSON string
valid events in the WebSocket API
-open
-close
-error
-message
Which Schedule APIs are supported by the Timer module in Node.js??
-setImmediate
-setInterval
-setTimeout
what is a RangeError ?
thrown when trying to pass a value as an arguement to a function that does not allow a range that includes the value
which is the first arguement typically passed to a callback handler in Node.js?
Error (object)
–> null if there is no error
push() method for arrays
method adds the specified elements to the end of an array and returns the new length of the array
What is target element?
and how can you access it?
the most deeply nested element that caused the event
accessible as –> event.target
What is event.currentTarget (=this)
the current element that handles the event (the one that has the handler on it)
–> so event.target is the most deeply nested element, but event.currentTarget keeps track of the current element as the event make its way through the capturing, target, and bubbling phase on the elements
.length() gives the length of what data type
strings
–> using .length() on a number value would return undefined
if you are sorting an array….where are the undefined elements sorted to?
end of the array
what is Promise.race() ?
static method that takes an iterable (ie array) of promises as input and returns a single promise. This returned promise settles with the eventual state of the first promise that settles
Window methods
*just read this one
Window.alert() - display an alert dialog
Window.close() - close the current window
Window.confirm() - display a dialog with a message that the user needs to respond to
Window.find() - search for a given string in a window
Window.focus() - set focus on the current window
Window.blur() - set focus away from the window
Window.open() - open a new window
Window.print() - open the Print Dialog to print the current document
Window.prompt() - return the text entered by the user in a prompt dialog
Window.resizeTo() - dynamically resize window
Window.scrollTo() - scroll to a particular set of coordinates in the document
Window.stop() - stop window loading
some callback characteristics?
-callback will never be called before the completion of the current run of the JavaScript event loop
function types related to async?
*think .then() .catch() .finally() for promises
try{}
catch{}
finally{}
name some of the functions that exist in the Promise API?
Promise.all()
Promise.allSettled()
Promise.race()
Promise.any()
fetch() API characteristics:
-returns a Promise that resolves to a Response object
-supports different request methods such as GET,POST,etc
-can take two arguements (1—> path to the resource 2–> optional which allows you to specify an object containing any custom settings you want to apply to the request…such as request method, headers, etc)
*what happens if the values used for a && operand are not booleans?
if operator is used with non-boolean values –> return the last non-boolean value
ex:
let a=5;
let b=20;
let c = (b && a); –> here c=5
how does nullish coalescing operator ?? work?
returns right hand side when –> left hand side operator is null or undefined
otherwise returns the left hand side
what is the result of num and why:
var num = 5n / 2n ;
2n
why–> BigInt results are truncated automatically, that’s why result is 2n instead of 2.5n
*also, fyi BigInt’s cant be mixed in operations with other types –> will produce a TypeError if you try
when using the super() keyword, what must you do??
use it at the beginning of the method, before accessing this or returning from the derived parent super constructor
*otherwise you will get a ReferenceError
when adding a variable that is undefined, the result of the addition will be =
NaN
how/functions to use to start a timer + check time elapsed since timer started
start timer –> console.time()
check timer –> console.timeEnd()
*time is outputted in miliseconds fyi
Error Types :
*just read this, top 5 most important
EvalError - Creates an instance representing an error that occurs regarding the global function eval().
RangeError - Creates an instance representing an error that occurs when a numeric variable or parameter is outside its valid range.
ReferenceError - Creates an instance representing an error that occurs when de-referencing an invalid reference.
SyntaxError - Creates an instance representing a syntax error.
TypeError - Creates an instance representing an error that occurs when a variable or parameter is not of a valid type.
URIError - Creates an instance representing an error that occurs when encodeURI() or decodeURI() are passed invalid parameters.
AggregateError - Creates an instance representing several errors wrapped in a single error when multiple errors need to be reported by an operation, for example by Promise.any().
InternalError Non-standard - Creates an instance representing an error that occurs when an internal error in the JavaScript engine is thrown. E.g. “too much recursion”.
if you initialize an array with the Array() keyword, and only put in one number without quotes like Array(2) what happens?
it creates an empty Array of length=2
if you want an array with a single value of 2, use quotes like this
const arr = new Array(‘2’)
is JavaScript case-sensitive?
yes
define these Array functions (only some of them)
findIndex()
find()
indexOf()
findIndex() –> returns the index of the first element in the array that satisfies the provided testing function
find() –> returns the value of the first element in the provided array that satisfies the provided testing function
indexOf() –> method returns the first index at which a given element can be found in the array
ex:
arr.findIndex( num => num > 13)
how many constructors can a class have in JavaScript?
only 1
*there is no constructor overloading in JavaScript –> only can have 1 constructor…….–> otherwise throws SyntaxError
correct way to get to get an extension of a file?
path.extname(‘main.js’)
object.seal()
method seals an object
–>new properties cannot be added
–>existing properties cannot be deleted
–> prototype cannot be reassigned
–> existing writable fields can be changed in value
Array Static methods
Array.from() - Creates a new Array instance from an iterable or array-like object.
Array.isArray() - Returns true if the argument is an array, or false otherwise.
Array.of() - Creates a new Array instance with a variable number of arguments, regardless of number or type of the arguments.
what does getDay() method return?
day of the week:
Sunday = 0
Saturday = 6
variable naming rules in JavaScript?
-first character must not be a digit
-must contain only letters, digits, or the symbols $ or _
var variable declaration in relation to hoisting
variables declared using var are created before any code is executed in a process known as hoisting
–> their initial value = undefined
Comparison between Map and Object
Object:
- Keys must be either a String or a Symbol.
- Keys are unordered.
- Size must be determined manually (use length is incorrect).
- Not optimized for frequent key-value pairs’ additions and removals.
Map:
- Keys can be any value (including function, objects, or primitives).
- Keys are ordered.
- Size can be determined from size property.
- Optimized for frequent key-value pairs’ additions and removals.
can date values for date objects be overflown?
yes, overflown is basically what it sounds like (putting larger numbers in)
ex:
let date = new Date(2020,20,34);
console.log(date.toString());
outputs: “Mon Sep 04 2021”
*Date(year, monthIndex [, day [, hours [, minutes [, seconds [, milliseconds]]]]])
what does the (…) spread syntax do?
allows expanding an iterable object (like an array) into the list of arguements when a list of arguements is expected
ex:
let arr = [8, 9, 15];
let merged = [0, …arr]
console.log(merged)
output–> [0, 8, 9, 15]
…arr spreads the array [8, 9, 15] into a list of arguments 8, 9, 15
what does !! do?
converts a non-boolean type into boolean type
*truthy / falsey value
what does
(1 && 2 && 3) return?
3
*last operand that is not evaluated as falsy
The result of a ?? b is:
*explain in basic terms how it works (Nullish coalescing operator)
if a is defined –then–> a
if a isn’t defined –then–> b
how can we define a private class field in JavaScript?
use a hash # symbol prefix
What is the only mathematical operator that can be used with strings?
+ operator (has a binary and unary form)
*everything else like - and others cannot be used with string
ex:
console.log( typeof (1 + “” + 0)) –> = string “10”
console.log(typeof (1 + “” - 0 )) —> = number 1
the two “special quirks” of the typeof operator?
-
typeof null –> = “object”
–> result of typeof null is “object”. That’s an officially recognized error in typeof, coming from very early days of JavaScript and kept for compatibility. Definitely, null is not an object. It is a special value with a separate type of its own. The behavior of typeof is wrong here. -
typeof alert –> = “function”
–> result of typeof alert is “function”, because alert is a function. We’ll study functions in the next chapters where we’ll also see that there’s no special “function” type in JavaScript. Functions belong to the object type. But typeof treats them differently, returning “function”. That also comes from the early days of JavaScript. Technically, such behavior isn’t correct, but can be convenient in practice.
What can you do with the “os” module in NodeJS?
-find out the “load average”
-return the host name
-return information on the CPUs available on your system
How do Equality Checks and Comparisons treat Null values
The equality check == and comparisons > < >= <= work differently.
Comparisons convert null to a number, treating it as 0. Hence, null >= 0 returns true
On the other hand, the equality check null == 0 does not get converted to a number, hence null == 0 is false.
Difference between Attributes and Properties?
Attributes – is what’s written in HTML.
Properties – is what’s in DOM objects.
What are the benefits of event delegation in JavaScript?
-lesser code is needed to add or remove the handlers
-event delegation simplifies the initialization as we don’t need to add multiple handlers to targeted elements
-Mass adding or removing the handlers on elements can be done easily
the BOM (Browser Object Model) consists of :
- window
- history
- navigator
- screen
- location
the DOM (Document Object Model) consists of:
- document
- element
correct way of initializing a XMLHttpRequest (*just read this one)
let xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
// Typical action to be performed when the document is ready
}
};
xhttp.open(“GET”, “https://google.com/search”, true);
xhttp.send();
instance methods for Date object
