·
Install cli
npm
install -g @angular/cli
·
uninstall cli
npm uninstall -g @angular/cli
npm cache clean
·
if cache clear fail run the command with
npm cache
clean –force
·
msinfo32
from run command gives answer on whether OS is 32 bit or 64 bit.
·
Opening control panel from command prompt appwiz.cpl
·
If ng command is not recognized then go to
“C:\Users\shahanu1\AppData\Roaming\npm”
for ng.cmd file. Copy the path and add to the environment variable.
Opening of system properties from command prompt: sysdm.cpl
·
Add path “C:\Program Files\Microsoft VS Code“ to
path variable in sysdm.cpl
·
Sysdm.cplàEnvironment
variableàSystem
VariablesàPathàNew Add new variables.
·
Upgrading CLI à “npm install -g @angular/cli@6” it upgrades cli to version 6.0
Steps to be able to create
angular apps in different version of angular
·
Get the latest CLI and install at global
level
·
In order to create angular app in older
version for example version 6 install CLI 6 in local folder.
·
To run angular app with ng command on a any
machine other than lcoalhost.
ng serve --port 8080
--host 0.0.0.0 --disableHostCheck true
also you can modify angular.json file.
ading serve on
.angular-cli.json.
Port here
4201. Solution found from #2542.
"defaults":
{
"styleExt":
"css",
"component":
{},
"serve":
{ "host": "0.0.0.0", "port": 4201 }
}
---------------------------------------------------------------------------------------
1)
run
command-" ng serve --host=0.0.0.0 --disable-host-check "
2)
This
will disable host check and allow to access from outside(instead localhost)
with ip address
Getting IP address of
Wifi
goto
system tray "All Settings" --> Network and Internet --> WiFi
--> Show available networks --> goto connected -->
if
its ethernet or wifi goto IPv4 address it is usually 192.168.32.x
Update npm 5:
As of npm 5.0.0, installed modules are
added as a dependency by default, so the
--save option
is no longer needed. The other save options still exist and are listed in
the documentation for npm install.
Original answer:
Before version 5, NPM simply installed a package under
node_modules by
default. When you were trying to install dependencies for your app/module, you
would need to first install them, and then add them (along with the appropriate
version number) to the dependencies section
of your package.json.
The
--save option instructed NPM to include the package inside of
the dependencies section of your package.json automatically, thus saving you an
additional step.
In addition, there are the complementary options
--save-dev and --save-optional which
save the package under devDependencies and optionalDependencies, respectively.
This is useful when installing development-only packages, like grunt or
your testing library.
install
Bootstrap 4 in Angular CLI with SASS (SCSS)
First we create a new project and
tell angular to use SCSS for styling.Open up your command prompt and enter:
ng new my-app --style=scss
cd my-app
Next we install Bootstrap 4 and the
optional modules with npm
npm install jquery --save-prod
npm install popper.js --save-prod
npm install bootstrap --save-prod
Create an empty file _variables.scss in src/.
This _variables.scss file you can use to
overwrite the Bootstrap variables. For more info on theming Bootstrap, read here.
In your project in src/styles.scss add
the following:
@import 'variables';
@import '~/bootstrap/scss/bootstrap';
In angular.json (Angular
6 and higher) add the three js files under scripts:
"scripts": [
"node_modules/jquery/dist/jquery.slim.js",
"node_modules/popper.js/dist/umd/popper.min.js",
"node_modules/bootstrap/dist/js/bootstrap.min.js"
],
Restart ng serve to see the changes.
Install Bootstrap
4 in Angular CLI with CSS
Create a new Angular CLI project
ng new my-app
cd my-app
Install bootstrap 4 with npm:
npm install jquery --save-prod
npm install popper.js --save-prod
npm install bootstrap --save-prod
Open the styles.css file
in your project.
Import the bootstrap style sheet as the
first line.
/* You can add global styles to this file, and also import other style files */
@import "~bootstrap/dist/css/bootstrap.min.css";
Please note that before Angular 6 you
would add the bootstrap css file to the styles section in the .angular-cli
file. This no longer works.
Open angular.json in
your project.
Find the first styles section
under the build node and add the following lines:
NOTE: there are two styles sections!! One
under build and one under test. If necessary put these lines in both sections.
"styles": [
"node_modules/bootstrap/dist/css/bootstrap.min.css",
"src/styles.css"
],
Next find the first scripts
section under the build node and add the following three scripts:
"scripts": [
"node_modules/jquery/dist/jquery.slim.js",
"node_modules/popper.js/dist/umd/popper.min.js",
"node_modules/bootstrap/dist/js/bootstrap.min.js"
],
Restart ng serve to see the changes.
Source:
- https://github.com/angular/angular-cli/wiki/stories-global-lib
- https://github.com/angular/angular-cli/wiki/stories-include-bootstrap
-----------------------------------------------------------------------------------------------------------------
IIS - URL rewrite module
Download
build
the angular app with
ng
build –base-ref “/<folder in inetpub\wwwroot>” –prod
for
deploying application at root level you copy content at
ng build –prod
Ping SSo
pickupLocBaseUrl
-https://ssolab.esurance.com/ext/ref/pickup
pickupLocBaseUrl
-https://ssolab.esurance.com/ext/ref/pickup?REF=E6FD8C9E1A5623EAFA9694EA18A484E279A9EC91D89336DB854714D00003
pUsername - PingFedAdaptor-lab
pPassword - E$ur4nc3!
application instanceid –
XPMAgentless
getmac address
cmd.exe
getmac /v /fo list
• opening programs - control appwiz.cpl
• admin -
control admintools
• printers - control printers
• services - services.msc
Reactive
forms provide a relatively easy testing strategy because they provide
synchronous access to the form and data models, and they can be tested without
rendering the UI.
Compodoc: Compodoc is a documentation tool for Angular
applications. It generates a static documentation of your application.
Steps
1)
npm install -g
"@compodoc/compodoc"
2)
npm install --save-dev
@compodoc/compodoc
3)
Define
a script task for it in your package.json
"scripts": {
"compodoc": "./node_modules/.bin/compodoc -p
src/tsconfig.app.json –w -s"
}
Run below command in a second terminal in VS code.
npm run compodoc
In order to add markdown
file (.md extension) for a “README” tab of a given component just add <name
of component file>.md in the component folder.
Hosting simple html page with live-server.
1)
Install nodejs
2) Npm
install –g live-server.
3)
Add path of live server which
generally is “C:\Users\shahanu1\AppData\Roaming\npm\node_modules\live-server\live-server.js”
to user variable.
4)
Cd <myfolder> folder where you
have index.html file
5) Type
command live-server.
6)
It will launch your html application.
Hurray.
You also may have noticed that we passed along
{ useHash:
true } as the second argument — by default Angular 2 uses
HTML5 location based routing, but that would require us to have a server set up
that's smart enough to handle it. For the sake of simplicity we told Angular 2
to use hash based routing instead as it's guaranteed to work in virtually any
environment setup.
Reactive
programming is the idea we can define an application as a series
of different streams with operations that connect the different
streams together and which are automatically called when new values are
pushed onto those streams.
Some people
claim that Observables are asynchronous. That is not true. If you surround a
function call with logs, like this:
Which proves the subscription
of foo was entirely synchronous, just like a function.
Observables
are able to deliver values either synchronously or asynchronously.
What is the difference between an
Observable and a function? Observables can "return" multiple
values over time, something which functions cannot. You can't do this:
Checking process listening port.
netstat -aon |find /i "listening”
Deploying Angular app to githubpages.
Prerequisites
You need to have Node.js, NPM and
Angular CLI 6 installed, you also need to generate a new Angular 6 project
using Angular CLI 6 or you can also apply this to an existing project that you
want to deploy to Github Pages.
Next you also need to install a
package which makes the process of deploying to Github pages easy.
$
npm install -g angular-cli-ghpages
Building the Angular 6 Application
The next step is to build your web
application for production and with the base href location set to your Github
Page URL.
$
ng build --prod --base-href
"https://<YOUR_GITHUB_USER-NAME>.github.io/<YOUR_REPO_NAME>/"
ng
build --prod --output-path <your_repo_name>
--base-href="https://<your_github_user-name>.github.io/<your_repo_name>/"
Deploying to Github Pages
Now you are ready to deploy your web
app to Github pages. Thanks to angular-cli-ghpages this is a one command
process so just run:
$
ngh
Providers –
In this case, providedIn: 'root'
specifies that the service should be provided in the root injector.
providedIn and NgModules
It's also possible to specify that a
service should be provided in a particular @NgModule. For example, if you don't
want UserService to be available to applications unless they import a
UserModule you've created, you can specify that the service should be provided
in the module:
The example above shows the
preferred way to provide a service in a module. This method is preferred
because it enables tree-shaking of the service if nothing injects it. If it's
not possible to specify in the service which module should provide it, you can
also declare a provider for the service within the module:
In
the basic CLI-generated app, modules are eagerly loaded which means that they
are all loaded when the app launches. Angular uses an injector system to make
things available between modules. In an eagerly loaded app, the root application injector
makes all of the providers in all of the modules available throughout the app.
This
behavior necessarily changes when you use lazy loading. Lazy loading is when you load
modules only when you need them; for example, when routing. They aren’t loaded
right away like with eagerly loaded modules. This means that any services
listed in their provider arrays aren’t available because the root injector
doesn’t know about these modules.
When the Angular router lazy-loads a
module, it creates a new injector. This injector is a child of the root
application injector. Imagine a tree of injectors; there is a single root
injector and then a child injector for each lazy loaded module. The router adds
all of the providers from the root injector to the child injector. When the router
creates a component within the lazy-loaded context, Angular prefers service
instances created from these providers to the service instances of the
application root injector.
Any
component created within a lazy loaded module’s context, such as by router navigation,
gets the local instance of the service, not the instance in the root
application injector. Components in external modules continue to receive the
instance created for the application root.
Though
you can provide services by lazy loading modules, not all services can be lazy
loaded. For instance, some modules only work in the root module, such as the
Router. The Router works with the global location object in the browser.
forRoot() and forChild()
You
might have noticed that the CLI adds RouterModule.forRoot(routes) to the
app-routing.module.ts imports array. This lets Angular know that this module,
AppRoutingModule, is a routing module and forRoot() specifies that this is the
root routing module. It configures all the routes you pass to it, gives you
access to the router directives, and registers the RouterService. Use forRoot()
in the AppRoutingModule—that is, one time in the app at the root level.
The
CLI also adds RouterModule.forChild(routes) to feature routing modules. This
way, Angular knows that the route list is only responsible for providing
additional routes and is intended for feature modules. You can use forChild()
in multiple modules.
forRoot() contains injector
configuration which is global; such as configuring the Router. forChild() has
no injector configuration, only directives such as RouterOutlet and RouterLink.
SkipSelf
decorator
A constructor parameter decorator
that tells the DI framework that dependency resolution should start from the
parent injector.
Optional
decorator
A constructor parameter decorator
that marks a dependency as optional.
importing a module does not
automatically re-export the imported module's imports. Module 'B' can't use
ngIf just because it imported module 'A' which imported CommonModule. Module
'B' must import CommonModule itself.
Re-export makes module transitivity
explicit. If Module 'A' re-exports CommonModule and Module 'B' imports Module
'A', Module 'B' components can use ngIf even though 'B' itself didn't import
CommonModule.
entryComponents
A list of components that can be dynamically loaded into the view.
By default, an Angular app always has at least one entry component, the
root component, AppComponent. Its purpose is to serve as a point of entry into
the app, that is, you bootstrap it to launch the app.
Routed components are also entry components because they need to be
loaded dynamically. The router creates them and drops them into the DOM near a
<router-outlet>.
While the bootstrapped and routed components are entry components, you
don't have to add them to a module's entryComponents list, as they are added
implicitly.
Angular automatically adds components in the module's bootstrap and
route definitions into the entryComponents list.
That leaves only components bootstrapped using one of the imperative
techniques, such as ViewComponentRef.createComponent() as undiscoverable.
Dynamic component loading is not common in most apps beyond the router.
If you need to dynamically load components, you must add these components to
the entryComponents list yourself.
For more information, see Entry
Components.
Bootstraping angular app using ngDoBootstrap
Hook
for manual bootstrapping of the application instead of using bootstrap array in
@NgModule annotation.
class
AppModule implements DoBootstrap {
ngDoBootstrap(appRef: ApplicationRef) {
appRef.bootstrap(AppComponent); // Or some
other component
}
}
WebSocket
To begin the process of repurposing
the TCP socket for WebSocket communication, the client can include a standard
request header that was invented specifically for this kind of use case:
GET /index.html HTTP/1.1
Host: www.example.com
Connection: Upgrade
Upgrade: websocket
The Connection header tells the
server that the client would like to negotiate a change in the way the socket
is being used. The accompanying value Upgrade indicates that the transport
protocol currently in use via TCP should change. Now that the server knows that
the client wants to upgrade the protocol currently in use over the active TCP
socket, the server knows to look for the corresponding Upgrade header, which
will tell it which transport protocol the client wants to use for the remaining
lifetime of the connection. As soon as the server sees websocket as the value
of the Upgrade header, it knows that a WebSocket handshake process has begun.
Note that the handshake process
(along with everything else) is outlined in RFC 6455, if you’d like to go into
more detail than is covered in this article.
The first part of the WebSocket
handshake, other than what is described above, involves proving that this is
actually a proper WebSocket upgrade handshake and that the process is not being
circumvented or emulated via some kind of intermediate trickery either by the
client or perhaps by a proxy server that sits in the middle.
When initiating an upgrade to a
WebSocket connection, the client must include a Sec-WebSocket-Key header with a
value unique to that client. Here’s an example:
Sec-WebSocket-Key: BOq0IliaPZlnbMHEBYtdjmKIL38=
The above is automatic and handled
for you if using the WebSocket class provided in modern browsers. You need only
look for it on the server side and produce a response.
When responding, the server must
append the special GUID value 258EAFA5-E914-47DA-95CA-C5AB0DC85B11 to the key,
generate a SHA-1 hash of the resultant string, then include it as the
base-64-encoded value of a Sec-WebSocket-Accept header that it includes in the
response:
Sec-WebSocket-Accept: 5fXT1W3UfPusBQv/h6c4hnwTJzk=
In a Node.js WebSocket server, we
could write a function to generate this value like so:
JS
const crypto = require('crypto');
function generateAcceptValue
(acceptKey) {
return crypto
.createHash('sha1')
.update(acceptKey + '258EAFA5-E914-47DA-95CA-C5AB0DC85B11', 'binary')
.digest('base64');
}
We’d then need only call this
function, passing the value of the Sec-WebSocket-Key header as the argument,
and set the function return value as the value of the Sec-WebSocket-Accept
header when sending the response.
To complete the handshake, write the
appropriate HTTP response headers to the client socket. A bare-bones response
would look something like this:
HTTP/1.1 101 Web Socket Protocol Handshake
Upgrade: WebSocket
Connection: Upgrade
Sec-WebSocket-Accept: m9raz0Lr21hfqAitCxWigVwhppA=
We’re not actually quite finished
with the handshake at this point – there are a couple more things to think
about.
WebSocket
messages are delivered in packages called “frames”, which begin with a message
header, and conclude with the “payload” – the message data for this frame.
Large messages may split the data over several frames, in which case you need
to keep track of what you’ve received so far and piece the data together once
it has all arrived.
Running Angular app using http-server –
·
Install http-server globally using
“npm install –g http-server”.
·
Goto dist folder of your angular
app. CD /dist/<angular-app>
·
Start the http-server using the
command on port 8081 “http-server –p
8081 –o” (-o option will launch the browser window)
·
Using the async pipe
The above is a good start but we are not really using Observables to their full extent, for one thing
we are subscribing to the observable and storing the results
locally on the component, we can skip
that by just using the async pipe in our template like
so:
class AppComponent {
private loading: boolean = false;
private results:
Observable<SearchItem[]>;
constructor(private itunes:SearchService) { }
doSearch(term:string) {
this.loading = true;
this.results = this.itunes.search(term); ①
}
}
① results now stores the Observable itself
and not an array of SearchItems. We don’t need to
subscribe but simply store the result of calling itunes.search.
To make the above work we need to use the async pipe in our template, like so:
<div
*ngFor="let item of results | async">
... </div>
What is the purpose of cache busting and how can you
achieve it?
Browsers have a cache to
temporarily store files on websites so they don't need to be re-downloaded
again when switching between pages or reloading the same page. The server is
set up to send headers that tell the browser to store the file for a given amount
of time. This greatly increases website speed and preserves bandwidth.
However, it can cause problems
when the website has been changed by developers because the user's cache still
references old files. This can either leave them with old functionality or
break a website if the cached CSS and JavaScript files are referencing elements
that no longer exist, have moved or have been renamed.
Cache busting is the process
of forcing the browser to download the new files. This is done by naming the
file something different to the old file.
A common technique to force
the browser to re-download the file is to append a query string to the end of
the file.
·
src="js/script.js" =>
src="js/script.js?v=2"
The browser considers it a
different file but prevents the need to change the file name.
What is BEM (Block Element Modifer)
#div1{
box-sizing: border-box;
width: 150px;
height: 200px;
padding: 5px;
border: 2px solid blue;
margin: 10px;
}
Below css styling - resets all of
the selected element's properties to their initial values as defined in the CSS
spec.
<div style="all:initial;">
<div id="div1">something</div>
</div>
NodeJS often uses a callback
pattern where if an error is encountered during execution, this error is passed
as the first argument to the callback. What are the advantages of this pattern?
fs.readFile(filePath,function(err,data){
if(err){
// handle the error, the return is important here
// so execution stops here
returnconsole.log(err)
}
// use the data object
console.log(data)
})
Hide answer
Advantages include:- Not
needing to process data if there is no need to even reference it
- Having
a consistent API leads to more adoption
- Ability
to easily adapt a callback pattern that will lead to more maintainable
code
As you can see from below
example, the callback is called with null as its first argument if there is no
error. However, if there is an error, you create an Error object, which then
becomes the callback's only parameter. The callback function allows a user to
easily know whether or not an error occurred.
This practice is also called
the Node.js error convention, and this kind of callback
implementations are called error-first callbacks.
varisTrue=function(value,callback){
if(value===true){
callback(null,"Value was true.")
}else{
callback(newError("Value is not true!"))
}
}
varcallback=function(error,retval){
if(error){
console.log(error)
return
}
console.log(retval)
}
isTrue(false,callback)
isTrue(true,callback)
/*
{ stack: [Getter/Setter],
arguments: undefined,
type: undefined,
message: 'Value is not true!' }
Value was true.
*/
Good to hear
What is the difference between null and undefined?
In JavaScript, two values
discretely represent nothing -
undefined and null. The concrete difference between them is that null is explicit, while undefined is implicit. When a property does not exist or
a variable has not been given a value, the value is undefined. null is set as the value to explicitly indicate “no
value”. In essence, undefined is used when the nothing is not known, and null is used when the nothing is known.
Good to hear
·
typeof undefined evaluates to "undefined".
·
typeof null evaluates "object". However, it is still a primitive value and this is
considered an implementation bug in JavaScript.
·
undefined == null evaluates to true.
Does JavaScript pass by value or by reference?
JavaScript
always passes by value. However, with objects, the value is a reference to the
object.
Good to hear
·
Difference between pass-by-value and pass-by-reference
How does prototypal inheritance differ from classical
inheritance?
In the classical inheritance paradigm, object instances inherit
their properties and functions from a class, which acts as a blueprint for the
object. Object instances are typically created using a constructor and
the
In the
prototypal inheritance paradigm, object instances inherit directly from other
objects and are typically created using factory functions or new keyword.Object.create().
What is the output of the following code?
consta=[1,2,3]
constb=[1,2,3]
constc="1,2,3"
console.log(a==c)
console.log(a==b)
The first
console.log outputs true because
JavaScript's compiler performs type conversion and therefore it compares to
strings by their value. On the other hand, the second console.log outputs false because Arrays
are Objects and Objects are compared by reference.
Good to hear
·
JavaScript performs automatic type conversion
·
Objects are compared by reference
·
Primitives are compared by value
Where and why is the rel="noopener" attribute
used?
The rel="noopener" is an attribute
used in <a> elements (hyperlinks). It prevents pages from having
a window.opener property, which would otherwise point to the page from
where the link was opened and would allow the page opened from the hyperlink to
manipulate the page where the hyperlink is.
Good to hear
·
rel="noopener" is applied to hyperlinks.
·
rel="noopener" prevents opened links from
manipulating the source page.
What is the difference between synchronous and asynchronous code
in JavaScript?
Synchronous
means each operation must wait for the previous one to complete.Asynchronous means an operation can occur while another operation is still being processed.
In JavaScript, all code is synchronous due to the single-threaded nature of it. However, asynchronous operations not part of the program (such as
XMLHttpRequest or setTimeout) are processed outside
of the main thread because they are controlled by native code (browser APIs),
but callbacks part of the program will still be executed synchronously.
Good to hear
·
JavaScript has a concurrency model based on an "event
loop".
·
Functions like
alert block the main thread so that no user input is registered
until the user closes it.
What are the differences between var, let, const and
no keyword statements?
When no keyword
exists before a variable assignment, it is either assigning a global variable
if one does not exist, or reassigns an already declared variable. In non-strict
mode, if the variable has not yet been declared, it will assign the variable as
a property of the global object (window in browsers). In strict mode, it will throw an error to
prevent unwanted global variables from being created.
var
var was the default
statement to declare a variable until ES2015. It creates a function-scoped
variable that can be reassigned and redeclared. However, due to its lack of
block scoping, it can cause issues if the variable is being reused in a loop
that contains an asynchronous callback because the variable will continue to
exist outside of the block scope.Below, by the time the the
setTimeout callback executes, the loop has already finished and
the i variable is 10, so all ten callbacks reference the same variable available in
the function scope.for(vari=0;i<10;i++){
setTimeout(()=>{
// logs `10` ten times
console.log(i)
})
}
/* Solutions with `var` */
for(vari=0;i<10;i++){
// Passed as an argument will use the value as-is in
// that point in time
setTimeout(console.log,0,i)
}
for(vari=0;i<10;i++){
// Create a new function scope that will use the value
// as-is in that point in time
;(i=>{
setTimeout(()=>{
console.log(i)
})
})(i)
}
let
let was introduced in
ES2015 and is the new preferred way to declare variables that will be
reassigned later. Trying to redeclare a variable again will throw an error. It
is block-scoped so that using it in a loop will keep it scoped to the
iteration.for(leti=0;i<10;i++){
setTimeout(()=>{
// logs 0, 1, 2, 3, ...
console.log(i)
})
}
const
const was introduced in
ES2015 and is the new preferred default way to declare all variables if they
won't be reassigned later, even for objects that will be mutated (as long as
the reference to the object does not change). It is block-scoped and cannot be
reassigned.constmyObject={}
myObject.prop="hello!"// No error
myObject="hello"// Error
Good to hear
·
All declarations are hoisted to the top of their scope.
·
However, with
let and const there is a concept called the temporal dead zone (TDZ).
While the declarations are still hoisted, there is a period between entering
scope and being declared where they cannot be accessed.
·
Show a common issue with using
var and how let can solve it, as
well as a solution that keeps var.
·
var should be avoided whenever possible and prefer const as the default
declaration statement for all variables unless they will be reassigned later,
then use let if so.
What is a cross-site scripting attack (XSS) and how do you
prevent it?
XSS refers to
client-side code injection where the attacker injects malicious scripts into a
legitimate website or web application. This is often achieved when the
application does not validate user input and freely injects dynamic HTML
content.For example, a comment system will be at risk if it does not validate or escape user input. If the comment contains unescaped HTML, the comment can inject a
<script> tag into the
website that other users will execute against their knowledge.- The malicious
script has access to cookies which are often used to store session tokens.
If an attacker can obtain a user’s session cookie, they can impersonate
the user.
- The script can
arbitrarily manipulate the DOM of the page the script is executing in,
allowing the attacker to insert pieces of content that appear to be a real
part of the website.
- The script can
use AJAX to send HTTP requests with arbitrary content to arbitrary
destinations.
Good to hear
·
On the client, using
textContent instead of innerHTML prevents the
browser from running the string through the HTML parser which would execute
scripts in it.
·
On the server, escaping HTML tags will prevent the browser from
parsing the user input as actual HTML and therefore won't execute the script.
How can you avoid callback hells?
getData(function(a){
getMoreData(a,function(b){
getMoreData(b,function(c){
getMoreData(c,function(d){
getMoreData(d,function(e){
// ...
})
})
})
})
})
Hide answer
Refactoring
the functions to return promises and using async/await is usually the
best option. Instead of supplying the functions with callbacks that cause deep
nesting, they return a promise that can be awaited and will be resolved
once the data has arrived, allowing the next line of code to be evaluated in a
sync-like fashion.The above code can be restructured like so:
asyncfunctionasyncAwaitVersion(){
consta=awaitgetData()
constb=awaitgetMoreData(a)
constc=awaitgetMoreData(b)
constd=awaitgetMoreData(c)
conste=awaitgetMoreData(d)
// ...
}
There are lots
of ways to solve the issue of callback hells:- Modularization:
break callbacks into independent functions
- Use a control
flow library, like async
- Use generators
with Promises
- Use async/await
(from v7 on)
Good to hear
·
As an efficient JavaScript developer, you have to avoid the
constantly growing indentation level, produce clean and readable code and be
able to handle complex flows.
What is a closure? Can you give a useful example of one?
A closure is a function defined inside another function and has
access to its lexical scope even when it is executing outside its lexical
scope. The closure has access to variables in three scopes:
- Variables
declared in its own scope
- Variables
declared in the scope of the parent function
- Variables declared
in the global scope
1) In
JavaScript, all functions are closures because they have access to the outer
scope, but most functions don't utilise the usefulness of closures: the
persistence of state. Closures are also sometimes called stateful functions because of this.
2) In
addition, closures are the only way to store private data that can't be
accessed from the outside in JavaScript. They are the key to the UMD (Universal Module Definition) pattern,
which is frequently used in libraries that only expose a public API but keep
the implementation details private, preventing
name collisions with other libraries or the user's own code.
Good to hear
·
Closures are useful because they let you associate data with a
function that operates on that data.
·
A closure can substitute an object with only a single method.
·
Closures can be used to emulate private properties and methods.
What is the difference between '+' and '~' sibling selectors?.
The
General Sibling Selector ~ selects all elements
that are siblings of a specified element.The following example selects all
<p> elements that are
siblings of <div> elements:div ~ p{
background-color:blue;
}
The
Adjacent Sibling Selector + selects all elements
that are the adjacent siblings of a specified element.The following example will select all
<p> elements
that are placed immediately after <div> elements:div + p{
background-color:red;
}
Additional links
Good to hear
·
ifference between '+' and '~' sibling selectors?.
·
Answer
·
The General Sibling Selector
~ selects
all elements that are siblings of a specified element.
·
The following example selects all
<p> elements
that are siblings of <div> elements:
·
`css
·
div ~ p {
·
background-color: blue;
·
}
·
`
·
The Adjacent Sibling Selector
+ selects
all elements that are the adjacent siblings of a specified element.
·
The following example will select all
<p> elements
that are placed immediately after <div> elements:
·
`css
·
div + p {
·
background-color: red;
·
}
·
`
Can you describe how CSS specificity works?
Assuming
the browser has already determined the set of rules for an element, each rule
is assigned a matrix of values, which correspond to the following from highest
to lowest specificity:- Inline rules
(binary - 1 or 0)
- Number of id
selectors
- Number of class,
pseudo-class and attribute selectors
- Number of tags
and pseudo-element selectors
Good to hear
·
Specificity matrix: [inline, id, class/pseudo-class/attribute,
tag/pseudo-element]
·
In cases of equal specificity, last rule is applied
What is a focus ring? What is the correct solution to handle
them?
A
focus ring is a visible outline given to focusable elements such as buttons and
anchor tags. It varies depending on the vendor, but generally it appears as a
blue outline around the element to indicate it is currently focused.In the past, many people specified
outline: 0; on the element to
remove the focus ring. However, this causes accessibility issues for keyboard
users because the focus state may not be clear. When not specified though, it
causes an unappealing blue ring to appear around an element.In recent times, frameworks like Bootstrap have opted to use a more appealing
box-shadow outline to replace the
default focus ring. However, this is still not ideal for mouse users.
The best solution is an
upcoming pseudo-selector
:focus-visible which
can be polyfilled today with JavaScript. It will only show a focus ring if the
user is using a keyboard and leave it hidden for mouse users. This keeps both
aesthetics for mouse use and accessibility for keyboard use.
What is a virtual DOM and why is it used in
libraries/frameworks?
The virtual
DOM (VDOM) is a representation of the real DOM in the form of plain JavaScript
objects. These objects have properties to describe the real DOM nodes they
represent: the node name, its attributes, and child nodes.<div class="counter">
<h1>0</h1>
<button>-</button>
<button>+</button>
</div>
The above
markup's virtual DOM representation might look like this:{
nodeName:"div",
attributes:{class:"counter"},
children:[
{
nodeName:"h1",
attributes:{},
children:[0]
},
{
nodeName:"button",
attributes:{},
children:["-"]
},
{
nodeName:"button",
attributes:{},
children:["+"]
}
]
}
The
library/framework uses the virtual DOM as a means to improve performance. When
the state of an application changes, the real DOM needs to be updated to
reflect it. However, changing real DOM nodes is costly compared to
recalculating the virtual DOM. The previous virtual DOM can be compared to the
new virtual DOM very quickly in comparison.Once the changes between the old VDOM and new VDOM have been calculated by the diffing engine of the framework, the real DOM can be patched efficiently in the least time possible to match the new state of the application.
Good to hear
·
Why accessing the DOM can be so costly.
CSS trick – image as base64 format
instead of path, url.
Did you know that you don't have to
link to an external image file when using an <img> element in HTML, or
declaring a background-image in CSS? You can embed the image data directly into
the document with data URIs.
With CSS, it looks like this:
CSS background:
HTML image:
Data-URI:
The biggest reason: it saves HTTP
Requests. Other than pure document size, this is the #1 factor concerning how
fast a page loads. Less = better.
CSS style to convert unordered list into 3 boxes equally
spaced.
<ul>
<li>1</li>
<li>2</li>
<li>3</li>
</ul>
CSS style.
ul>li{
display: inline;
border: 1px solid red;
padding: 10px;
margin-right: 5px;
}
- margin-top
- margin-right
- margin-bottom
- margin-left
TRBL
With the CSS box-sizing Property
·
The box-sizing property allows us to
include the padding and border in an element's total width and height.
·
If you set box-sizing: border-box;
on an element padding and border are included in the width and height:
·
The initial keyword is used to set a CSS
property to its default value.
The initial keyword can be used for
any CSS property, and on any HTML element.
Grid
.wrapper {
display: grid;
grid-template-columns: 200px auto 200px 200px;;
grid-template-rows:450px 250px auto;
grid-column-gap: 10px;
grid-row-gap: 20px;
justify-items: stretch;
align-items: start;
justify-content: space-evenly;
align-content: space-around;
}
* {box-sizing: border-box;}
.wrapper > div {
border: 10px solid #e0d1c0;
border-radius: 1px;
background-color: #ffd8a8;
padding: 1em;
color: #d9480f;
}
Declaring variable in CSS
:root{
--main-bg-color: brown;
}
.mycls{
background-color: var(--main-bg-color);
}
Closures Advantage.
·
Stateful functions
·
Encapsulating private data and
internal implementation.
·
Provide UMD pattern used in js
libraries, providing public APIs, preventing name collision with other
libraries and data.
·
Associate data with a functions that
operates on a data.
·
Can substitute an object with only a
single method.
CSS
@import "~bootstrap/dist/css/bootstrap.min.css";
ul>li {
display: inline-grid;
margin-right: 20px;
width: inherit;
height: inherit;
}
div>a {
display: inline;
}
div a h5 {
color: darkolivegreen;
}
a:hover {
text-decoration: none;
}
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