Javascript crash course in 2022

JavaScript Tutorial For Beginners Step by Step 2022

What is Javascript?

JavaScript is a scripting or programming language that allows you to implement complex features on web pages.

The programs in this language are called scripts. They can be written right in a web page’s HTML and run automatically as the page loads.

JavaScript can execute not only in the browser but also on the server.

The browser has an embedded engine sometimes called a “JavaScript virtual machine”.

For example:

V8 – in Chrome, Opera and Edge.

SpiderMonkey – in Firefox.

Popular Javascript Frameworks out there you may want to Explore…

  1. Node.js
  2. Vue.js
  3. Ember.js
  4. Angular.js
  5. React.js

Topics to cover:

  1. Variables and Constants
  2. Arrays and objects
  3. The Dom
  4. Functions
  5. Primitive Types

Variables

A variable is a “named storage” for data.

To create a variable in JavaScript, use the let keyword.

Ex:

let message;
message = 'Hello!';
alert(message); // shows the variable content

More Example programs:

let bucket;
bucket = 'Water';
bucket = 'Soil'; // value changes
alert(bucket);

or…

let admin, name;
name = "John";
admin = name;
alert(admin);

Rules to follow when naming variables:

  • The name must contain only letters, digits, or the symbols $ and _.
  • The first character must not be a digit.

Ex:
let firstName, lastName, userName etc. // referred to as camelCase

let $ = 1; // declared a variable with the name “$”

let _ = 2; // and now a variable with the name “_”

alert($ + _); // 3

Constants

a constant is an (unchanging) variable to declare we use const instead of let

const pi = 3.14;
pi = 2.44; // trying to reassign, error, can't reassign the constant!
alert(pi);

Uppercase Constants

const BIRTHDAY = ‘18.04.1982’; // make uppercase? yes, why -> We generally use upper case for constants that are

“hard-coded”. Or, in other words, when the value is known prior to execution and directly written into the code.

We can also have something like, const COLOR_WHITE = “#FFFF”;

let color = COLOR_WHITE;
alert(color); // #FFFF its easier to remember white than the hex value #fff

const age = someCode(BIRTHDAY); // make uppercase? (AGE) no, why -> In contrast, age is evaluated in run-time. Today we

have one age, a year after we’ll have another one. It is constant in the sense that it does not change through the code execution. But it is a bit “less of a constant” than birthday: it is calculated, so we should keep the lower case for it.

Arrays

An array is a data structure consisting of a collection of elements In other words Arrays to store ordered collections.

You can create an array with the following syntax:

let arr = [];
let arr = new Array(); // old fashioned rarely used

Ex:

let cars = ["Toyota", "Mercedes", "Tesla"]
alert( cars[0] ); // Toyota
alert( cars[1] ); // Mercedes

We can replace an element:

fruits[2] = 'Filda'; // now ["Toyota", "Mercedes", "Filda"]

…Or add a new one to the array:

fruits[3] = 'BMW'; // now ["Toyota", "Mercedes", "Filda", "BMW"]

The total count of the elements in the array is its length:

let fruits = ["Toyota", "Mercedes", "Filda"]
alert( fruits.length ); // 3

Methods pop/push

  1. push adds an element to the end.

Ex:

let fruits = ["Toyota", "Mercedes"];
fruits.push("Filda");
alert( cars ); // Toyota, Mercedes, Filda
  1. pop takes an element from the end.

Ex:

let cars = ["Toyota", "Mercedes", "Filda"]
alert( cars.pop() ); // remove "Filda" and alert it
alert( cars ); // Toyota Mercedes
  1. shift

Extracts the first element of the array and returns it:

Ex:

let cars = ["Toyota", "Mercedes", "Filda"]
alert( cars.shift() ); // remove Toyota and alert it
alert( cars ); // Mercedes, Filda
  1. unshift

Add the element to the beginning of the array

Ex:

let cars = ["Mercedes", "Filda"];
fruits.unshift('Toyota');
alert( fruits ); // Toyota, Mercedes, Filda

NB: Methods push/pop run fast, while shift/unshift are slow.

We can also use for loop to iterate over array elements

  1. For Loop:
Ex:
let arr = ["Toyota", "Mercedes", "Filda"];
for (let i = 0; i < arr.length; i++) {
alert( arr[i] );
}

Objects
objects are used to store keyed collections of various data.

Ex:
We can put properties into {…} as “key: value” pairs:

let user = {
name :"Brian",
surname : "Mweu",
//name : "Merinda"
};

Property has a key (also known as “name” or “identifier”) before the colon “:” and a value to the right of it.

Property values are accessible using the dot notation:

alert(user.name); // Brian

To remove a property, we can use the delete operator:

delete user.name; // undefined Brian has been deleted

The “for…in” loop

The syntax:

for (key in object) {
// executes the body for each key among object properties
}
Ex:
let fruit = {
name: "Mango",
color: "yellow",
price: 10,
};
for (let key in fruit) {
// keys
alert( key ); // name, color, price
// values for the keys
alert( fruit[key] ); // Mango, yellow, 10
}

Functions

Functions are the main “building blocks” of the program.

They allow the code to be called many times without repetition.

Function Declaration

To create a function we can use a function declaration

Ex:

function helloWorld() {
   alert( 'Hello world!' );
}

The analogy here is

 function name(parameter1, parameter2, … parameterN) {
      …body…
 }

we call the function using the function name as follows

helloWorld();

We can have local variables inside a function which means they are only local to the function

Ex:

function helloWorld() {
let message = "Hello world my name is Brian"
alert(message);
}
helloWorld();

//alert( message ); // error the variable is a local variable

As we have local variables we also have global variables in a function such as

let price = '20'; // global variable
function showPrice() {
let message = 'The price of a single mango is, ' + price;
alert(message);
}
showPrice();

Functions can have parameters as I described in the syntax above

Ex:

function checkAge(age) {
if(age >= 18) {
return true;
} else {
return confirm('You are under 18!')
}
}
let age = prompt("Enter your age", 18);
if ( checkAge(age) ) {
alert( 'Access granted' );
} else {
alert( 'Access denied' );
}

// Access granted

Functions can return a value back the above example shows it well. Remember we have named the functions within the function declaration I talked about at the start.

Primitives Types

JavaScript allows us to work with primitives (strings, numbers, etc.) as if they were objects.

Let’s look at the key distinctions between primitives and objects.

A primitive Is a value of a primitive type.

There are 7 primitive types: string, number, bigint, boolean, symbol, null and undefined.

An object Is capable of storing multiple values as properties.
Can be created with {}, for instance: {name: “John”, age: 30}. There are other kinds of objects in JavaScript: functions, for example, are objects

Ex:

let brian = {
name: "Brian",
sayHi: function() {
alert("Hi Brian!");
}
};

brian.sayHi(); // Hi Brian!

So here we’ve made an object john with the method sayHi.

A primitive as an object

For instance, there exists a string method str.toUpperCase() that returns a capitalized str.

Here’s how it works:

let str = "Brian";
alert( str.toUpperCase() ); // BRIAN

Simple, right? Here’s what actually happens in str.toUpperCase():

  1. The string str is a primitive. So in the moment of accessing its property, a special object is created that knows the value of the string, and has useful methods, like toUpperCase().
  2. That method runs and returns a new string (shown by alert).
  3. The special object is destroyed, leaving the primitive str alone.

So primitives can provide methods, but they still remain lightweight.

NB:
null/undefined has no methods

The special primitives null and undefined are exceptions. They have no corresponding “wrapper objects” and provide no methods. In a sense, they are “the most primitive”.

An attempt to access a property of such value would give the error:

alert(null.test); // error

Another Ex:

let name= "Brian";

console.log(Object.keys(name));
console.log(Object.values(name));

Output:
(5) ["0", "1", "2", "3", "4"]
(5) ["B", "r", "i", "a", "n"]

So that means we can use methods of Objects on primitives.

The DOM

The Document Object Model (DOM) is the data representation of the objects that comprise the structure and content of a document on the web.

It represents the page so that programs can change the document structure, style, and content.

This is just an introduction on getting started with the Dom

Ex:
For example, document.body is the object representing the tag.

index.html

<body>

<h1 style="text-align:center;">Welcome to Javascript</h1>

    <script type="text/javascript" src="script.js"></script>
    
</body>

Ex:

script.js

document.body.style.background = 'blue'; // make the background red

setTimeout(() => document.body.style.background = '', 4000); // return back

document.getElementById or just id

index.html

<div id="main">
    <div id="Main-content">Main Content</div>
  </div>
<script>
 // get the element
let elem = document.getElementById('main');

// make its background blue
elem.style.background = 'red';
</script>

querySelectorAll
Returns all elements inside elem matching the given CSS selector. Here we look for all elements that are last children:

Ex:
index.html

<ul>
    <li>Mango</li>
    <li>Oranges</li>
    <li>Bananas</li>
  </ul>

script.js

<script>
    let elements = document.querySelectorAll('ul > li:last-child');
  
    for (let elem of elements) {
      alert(elem.innerHTML); // "Bananas"
    }
  </script>

Modifying the Document

Create a Message

Creating the message div takes 3 steps:

1. Create an element


let div = document.createElement('div');

2. Set its class to “alert”


div.className = "alert";

3. Fill it with the content


div.innerHTML = "You've 4 unread Mail Notifications.";

Ex:
index.html

<style>
    .alert {
      padding: 15px;
      border: 1px solid #d6e9c6;
      border-radius: 4px;
      color: #3c763d;
      background-color: #dff0d8;
    }
    </style>

script.js


let div = document.createElement('div');
div.className = "alert";
div.innerHTML = "You've 4 unread Mail Notifications.";

document.body.append(div);
setTimeout(() => div.remove(), 1000); // you can remove the node and set the timeout

We will dig deeper into the Dom in the coming Javascript tutorials.

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