Function

strlen()

returns the length of the string

echo strlen("Hello world!")."\n"; // 12

Function

strrev()

returns the Reverse of the string

echo strrev("Hello world!")."\n"; // !dlrow olleH

Function

strpos()

tries to find the second parameter and returns its index

echo strpos("Hello world!", "world")."\n"; // 6

Function

str_replace()

replaces the word "Hello" with "Hi"

echo str_replace("Hello", "Hi", "Hello there")."\n"; // Hi there

Function

substr()

returns the substring between given indexes

echo substr("Hello world", 6)."\n"; // world
echo substr("Hello world", 6, 3)."\n"; // wor
echo substr("Hello world", -5)."\n"; // world
echo substr("Hello world", -4, 2)."\n"; // or

Function

trim()

removes whitespaces from sides of the string

echo trim("   Hello World  ").".\n"; // Hello World.
// Removes whitespaces from the right end
echo rtrim("   Hello World  ")."\n"; //    Hello World
// Removes Exclamation mark from the right end
echo rtrim("Hello World!", "!")."\n"; // Hello World

Function

strtoupper()

converts the string to upper case

echo strtoupper("Hello World")."\n";

Function

strtolower()

converts the string to lower case

echo strtolower("Hello World")."\n";

Function

explode()

breaks the string by its delimiter

print_r (explode(" ", "It's the first day of the course!"));

Function

implode()

joins the array element based on the delimiter and returns the string

echo implode(" ", ['Hello', 'World']);

The three variables

$variable

,

$Variable

and

$VARIABLE

are completely different

// lowercase
$variable = "Value 1";

// PascalCase
$Variable = "Value 2";

// UPPERCASE
$VARIABLE = "Value 3";

echo $variable . "\n";
echo $Variable . "\n";
echo $VARIABLE . "\n";

Function names are case-insensitive

function printGreeting() {
	print "Prints: Hello world!\n";
}
printGreeting();

Calling the function with uppercase will still work

Function declarations are hoisted. It means, we can use the function before we declare it

hoisted(); // logs "foo"

function hoisted() {
  console.log('foo');
}

Function expressions - anonymous functions are not hoisted:

notHoisted(); // TypeError: notHoisted is not a function

var notHoisted = function() {
   console.log('bar');
};

The

map

function calls

getNames

function for each item of the array. Each time the function

getNames

returns only names from each object

let myFavoritePlanets = [
    {name: 'Earth', radius: 6371}, 
    {name: 'Jupiter', radius: 69911}, 
    {name: 'Neptune', radius: 24622, moons: ['Triton', 'Thalassa', 'Nereid', 'Proteus', 'Hippocamp', 'Naiad']}
]
let namesOnly = myFavoritePlanets.map(getNames);

function getNames(planet) {
  return planet.name
}
console.log(namesOnly);

If a fatal error occurs catch block calls

logError

function and passes details of the error, such as:

file

,

line

,

code

,

message

and

trace

try {
    undefinedFunction();
} catch (Error $e) {
    logError(
		'ERR FILE: ' . $e->getFile() . "\n" .
		'ERR LINE: ' . $e->getLine() . "\n" .
		'ERR CODE: ' . $e->getCode() . "\n" .
		'ERR TEXT: ' . $e->getMessage() . "\n" .
		'ERR TRACE String: ' . $e->getTraceAsString()
	);
}

The function

logError

appends current date and time to the error details and logs into the error file

function logError($error) {
	
	$content = "\n\n\n".date('Y-m-d H:i:s');
	$content .= "\n".$error;
	
	file_put_contents('/var/log/errors.txt', $content, FILE_APPEND);
}

Let's declare any string variable

let myString = 'Some String'

The string variable has access to all

string functions

document.write(myString.toUpperCase());

The result will be:

SOME STRING

But if we let number variable to use the string function

toUpperCase()

let myNumber = 55
console.log(myNumber.toUpperCase());

Then we will get the following error. This is because number type variable does not have access to string functions

Uncaught TypeError: myNumber.toUpperCase is not a function

The function

addEventListener

creates the specified event listener for the object. In this case we add

click

event for the div element with id

div_element

without using anonymous function. First we select the object that we want to create

click

event listener to

let div = document.getElementById('div_element');

Then we create the actual event listener for the event without anonymous function

div.addEventListener('click', func);
function func() {
    alert('You clicked me!');
}

The same example using anonymous function

div.addEventListener('click', function(){
  alert('You clicked me!');
})

HTML part of the examples:

<div id="div_element">Click Me</div>

Another method to add the event listener is to include

onclick

attribute inside opening tag

<div onclick="func()">Click Me</div>

It's possible to have an object inside another object. In the example we have object

boyfriend

inside

cat

object

let cat = {
    name: 'Lucy',
    age: 4,
    meow() {
        console.log('Meowwww');
    },
    boyfriend: {
        name: 'Jack',
        age: 5,
        favoriteColors: ['Green', 'Yellow'],
        meow() {
            console.log('Meeoowwwwwwwwwwww');
        },
    }
}

Accessing nested object's attributes and methods:

console.log(cat.boyfriend.name); // Jack
cat.boyfriend.meow(); // Meeoowwwwwwwwwwww

The object

cat

combines attributes and behaviors of a cat in the following object

let cat = {
    age: 4,
    name: 'Bella',
    meow() {
        console.log('Meowwww');
    }
}

Instead of the object, we would have separated function and variables, which is not organized way

let catName = 'Bella';
let catAge = 4;

function meow() {
    console.log('Meowwww');
}

Sets

green

background color

document.body.style.backgroundColor = 'green';

Sets

display

property to

inline-block

for the element with id

element_id

document.getElementById('element_id').style.display = 'inline-block';