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Valeri Tandilashvili's Personal Professional Blog
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delete folder
Removes unwanted folder 
some_folder
sudo rm -r some_folder/
-r
means that it removes the directory with its all child directories
XML Tags Rules
XML tags are 
case-sensitive
- because of the case difference in two tags, which is treated as incorrect syntax in XML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
   <address>Wrong syntax</ADdress>
   <hr />
</student>
This will generate the error:
error on line 6 at column 23: Opening and ending tag mismatch: address line 0 and ADdress
XML tags must be closed 
in an appropriate order
, which means an XML element opened inside another element must be closed before the outer element
<outer_element>
   <internal_element>
      Right, because the inner tag is closed before the outer
   </internal_element>
</outer_element>
npm install --help
Shows all the different ways we can use 
npm install
command (in console)
npm install -h
Alternative to the above command
npm install --help
ampersand 
.alert {
  // The parent selector can be used to add pseudo-classes to the outer
  // selector.
  &:hover {
    font-weight: bold;
  }

  // It can also be used to style the outer selector in a certain context, such
  // as a body set to use a right-to-left language.
  [dir=rtl] & {
    margin-left: 0;
    margin-right: 10px;
  }

  // You can even use it as an argument to pseudo-class selectors.
  :not(&) {
    opacity: 0.8;
  }
}
This will generate the following regular CSS:
.alert:hover {
  font-weight: bold;
}
[dir=rtl] .alert {
  margin-left: 0;
  margin-right: 10px;
}
:not(.alert) {
  opacity: 0.8;
}
Inheritance / @extend
.btn-b extends .btn-a:
.btn-a {
    display: inline-block;
    padding: 10px;
}
.btn-b {
    @extend .btn-a;
    background-color: black;
}
Result in regular CSS:
.btn-a, .btn-b {
    display: inline-block;
    padding: 10px;
}
.btn-b {
    background-color: black;
}
abstract class can contain both methods with definitions and abstract methods 
abstract class Fruit { 
    private $color; 
    
    abstract public function eat(); 
    
    public function setColor($c) { 
        $this->color = $c; 
    } 
} 

class Apple extends Fruit {
    public function eat() {
        echo "Omnomnom";
    }
}

$obj = new Apple();
$obj->eat();
In this example an 
interface
would not be able to have method with definition, only method signatures are allowed in interfaces
letter-spacing, word-spacing
We can style text using 
letter-spacing
and 
word-spacing
properties to have some space between letters and words
h1 {
    letter-spacing: 2px;
    word-spacing: 10px;
}
col-sm grid system
The grid system that starts out stacked and becomes horizontal at the small breakpoint (sm)
<div class="container">
  <div class="row">
    <div class="col-sm">col-sm</div>
    <div class="col-sm">col-sm</div>
    <div class="col-sm">col-sm</div>
  </div>
</div>
container sizes with different devices CODE 
100%
Extra small <576px 
540px
Small ≥576px 
720px
Medium ≥768px 
960px
Large ≥992px 
1140px
Extra large ≥1200px
calling non-static methods statically is deprecated 
class Foo {

    private $color;

    public function bar() {
        echo 'before';
        $this->color = "blue";
        echo 'after';
    }
}

// Foo::bar();  

// Deprecated
$obj = new Foo;
$obj::bar();
Deprecated: 
Non-static method Foo::bar() should not be called statically
php.net: 
In PHP 7, calling non-static methods statically is deprecated, and will generate an E_DEPRECATED warning. Support for calling non-static methods statically may be removed in the future
Results: 1022