Regular anonymous function:

let numbers = [10, 100, 1000]

let doubled = numbers.map(function(x){
  return x * 2
})

document.write(doubled)

Alternative arrow function:

let doubled = numbers.map((x) => { return x * 2 })

If it's only a single line we don't need curly brackets and the keyword

return

let doubled = numbers.map((x) => x * 2);

If the arrow function has only one parameter, it does not need parentheses:

let doubled = numbers.map(x => x * 2);

Note: We only need

parentheses

if we had zero or more than one parameters

When we need to join a table to itself, it is called

SELF JOIN

SELECT  
    s1.*,
    s2.first_name
FROM `students` s1
JOIN `students` s2 ON s2.santa_id = s1.id

This type of join is often used when we have tree structure in a table. Note:

alias

is mandatory in

SELF JOIN

We can give

alias

to tables or table columns. In this example table column

first_name

will be displayed as

student_name

SELECT 
    id,
    first_name AS student_name
FROM students

The keyword

AS

is optional, but it's better to use it, because the query is much more readable. The following query returns the same result

SELECT 
    id,
    first_name student_name
FROM students

We can use

alias

in

ORDER BY

clause to order the result

SELECT *, price * quantity AS total
FROM `orders` 
ORDER BY total DESC

If

alias

is given to table, column of the table can be accessed by the alias followed by

.

SELECT 
    s.id,
    s.first_name
FROM `students` s

In

SELF JOIN

alias is required, because MySQL needs to treat the tables as different tables

SELECT  
    s1.*,
    s2.first_name
FROM `students` s1
JOIN `students` s2 ON s2.santa_id = s1.id

quotes

or backticks can be used in alias. Possible options: single quote

'

, double quote

"

or backtick

 ` 

SELECT
    id,
    first_name AS 'student_name'
FROM 
    students

Advantage of using

quotes

is that we can use space separated names in alias

SELECT 
    first_name, 
    last_name, 
    (points + 10) * 2 AS 'points calculated'
FROM students

The following example shows how to convert an array into JSON

$arr = array('a' => 1, 'b' => 2, 'c' => 3, 'd' => 4, 'e' => 5);

// Returns the JSON representation of a value (pretty printed)
echo json_encode($arr, JSON_PRETTY_PRINT);

Result of the above code

{
    "a": 1,
    "b": 2,
    "c": 3,
    "d": 4,
    "e": 5
}

The following example shows how the PHP objects can be converted into JSON

class Emp {
    public $name = "";
    public $hobbies  = [];
    public $birthdate = "";
}

$e = new Emp();
$e->name = "sachin";
$e->hobbies  = ["sport", "reading"];
$e->birthdate = date('Y-m-d', strtotime('2015-11-18'));

echo json_encode($e);

The above code will produce the following result

{"name":"sachin","hobbies":["sport","reading"],"birthdate":"2015-11-18"}

Decoding JSON in PHP (json_decode) The following example shows how PHP can be used to decode JSON objects

$json = '{"a":1,"b":2,"c":3,"d":4,"e":5}';

// Decoding into an object
var_dump(json_decode($json));

// Decoding into an array
var_dump(json_decode($json, true));

While executing, it will produce the following result

object(stdClass)#1 (5) {
  ["a"]=> int(1)
  ["b"]=> int(2)
  ["c"]=> int(3)
  ["d"]=> int(4)
  ["e"]=> int(5)
}
array(5) {
  ["a"]=> int(1)
  ["b"]=> int(2)
  ["c"]=> int(3)
  ["d"]=> int(4)
  ["e"]=> int(5)
}

There are 3 types of whitespace within an XML document, XML Syntax, Significant and Insignificant

1. XML Syntactical whitespace - is whitespace required by the XML in order to delimitate XML constructs.

2. Insignificant Whitespace - is whitespace, that is not considered part of an element, if two elements are separated by just whitespace its considered insignificant.

3. Significant Whitespace - is whitespace that XML element attribute contains.

this

inside

driveCar()

function refers to the global window object. Global variables

firstName

and

lastName

are added to the

window

object. That is why they are visible using the keyword

firstName = "Jack"
lastName = "Ma"

function driveCar() {
  document.write(this.firstName + ' ' + this.lastName + ' is driving a car<br/>')
}

driveCar()

If we call the same function using

call

function,

this

will refer to the object that will be passed. In this case:

john

object

let john = {
  firstName: 'John',
  lastName: 'Doe'
}

function driveCar() {
  document.write(this.firstName + ' ' + this.lastName + ' is driving a car<br/>')
}

driveCar.call(john)

Rules that should be followed when we use XML comments:

- Comment must appear after XML declaration.
- Comment may appear anywhere in a document
- Comment must not appear within attribute values.
- Comment cannot be nested inside the other comment.

In JavaScript the statement evaluates to

true

but in PHP it evaluates to

false

if ("0") {
    
}

To assign margin related properties using the classes:

.mt-3

- margin-top

.mr-3

- margin-right

.mb-3

- margin-bottom

.ml-3

- margin-left

.mx-3

- margin-left, margin-right

.my-3

- margin-top, margin-bottom

.m-3

- margin-top, margin-bottom, margin-left, margin-right The same for padding properties:

.pt-3

- padding-top

.pr-3

- padding-right

.pb-3

- padding-bottom

.pl-3

- padding-left

.px-3

- padding-left, padding-right

.py-3

- padding-top, padding-bottom

.p-3

- padding-top, padding-bottom, padding-left, padding-right

Function is not allowed in JSON. If we need to have a function in JSON, we have to include it as a string. Later, we can convert it back into a function

var text = '{ "name":"John", "age":"function () {return 30;}", "city":"New York"}';
var obj = JSON.parse(text);
obj.age = eval("(" + obj.age + ")");

document.getElementById("demo").innerHTML = obj.name + ", " + obj.age();