Basics
JavaScript is a versatile, cross-platform programming language widely used for web development. It allows developers to create interactive, dynamic web pages and applications. JavaScript is primarily used for client-side scripting, but it can also be used on the server side with environments like Node.js. JavaScript is an interpreted, high-level, dynamically typed, and general-purpose language. It supports procedural, object-oriented, and functional programming. It is primarily used for building websites, web applications, mobile applications, and server-side tools. The extension of a JavaScript file is .js.
We can write JavaScript code inside the <script> tags placed within the <body> section of an HTML document or in an external .js file. We connect the external file in the <head> section or at the bottom of the <body> section like this: <script src="script.js"></script>. If we just want to test some simple scripts, we can run them in the browser console by opening the "Console" tab from the developer tools. It's a quick way to run and debug scripts without needing an HTML file.
Fundamental rules
We count from zero, not from one.
Quotation marks and apostrophes mean the same, although we can't close one string between a quotation mark and an apostrophe.
In JavaScript, a semicolon can but doesn't have to be placed after each statement. We must indicate which instructions belong to, e.g., a loop, by putting them in braces (unless it's only one line).
We can create comments by using the // symbols. Things that we write after these signs don't count as code. Comments are often used to describe the code. Multiline comments can be written between /* and */ signs.
Variables
A variable is a "drawer" for data in the RAM. We can store every sort of data in them. We have five basic variable types:
int - integers
float - decimals
string - texts (a number can also be a part of it, e.g., "abc123")
bool (boolean) - true (all numbers except 0) and false (0) values
null - an object used to indicate the absence of a value.
undefined - the default value of uninitialized variables.
In JavaScript (unlike in, e.g., C++), we do not have to define the type when creating a variable. It is assigned automatically based on the value. Here, integers and floating-point numbers are all handled by the "number" type, however keep in mind that int and float operate "below." There is also a type called BigInt, and it is used for really long integers. We indicate its usage by adding an n at the end of the number, e.g., 1234567890123456789012345678901234567890n.
We can check the variable's type using the typeof x method.
Everything on the left side of the = sign is a variable's name (it cannot contain spaces), and on the right is a value.
Be cautious because, e.g., variables x and X are not the same!
let number = 4 will initialize an int variable called "number" and immediately assign it the value 4. number = 5 will change the value of this variable to 5. Instead of let, we can use var. A variable initialized with the former is a local variable and with the latter - a global variable (I will talk about it in the upcoming lessons).
If we don't immediately have any value to assign to the variable, we can simply declare it, e.g., let number2. We say that we declare a variable if it is empty at the beginning, and we initialize it when we give it a value. We can assign a value to the declared variable the same way as if we were changing the value of a variable that has already been initialized (number2 = 6).
While naming variables, we cannot add any numbers or spaces at the beginning. The first letter should not be in uppercase. The three correct ways of naming variables are: prime_number, primeNumber, nPrimeNumber (Camel case or Snake case).
Variables can be declared in bulk, e.g., let a, b, c. They can also be defined in the same way: let a = 1, b, c = 2 (we can define only some of them if we want).
We can define a variable as a constant, whose value cannot be changed, by adding const before the variable definition (e.g., const NAME = "text"). There is a convention that the names of constants should be in uppercase.
Variables in JavaScript can change their type during the program runtime (they are dynamically typed).
A "falsy" number is a number that, after conversion to a boolean, e.g., in a conditional statement, equals false, e.g., 0.
\n symbolizes the enter key and can be used inside of strings.
*In the examples I provide, I will not always give the entire code but fragments of it.
For now, a method is a special command that performs a specific task, often related to something in our program, like printing text or calculating a result (I will talk more about methods later).
Displaying data
# Displaying text in the console
console.log("Hello World!")
# Displaying text in the HTML document
document.write("Hello World!")
# Displaying text in a pop-up window
alert("Hello World!")
User input
# Displaying text in a OK / Cancel pop-up window and saving the boolean value in a variable
let value = confirm("Are you sure?")
# Displaying text in a pop-up window with a textbox and saving the string value in a variable
let value2 = prompt("What is your name?")
Type casting
Type casting, also known as type conversion, is the process of converting one data type to another (e.g., float to int).
let x = 5.5
let y = parseInt(x) // there is also parseFloat(), etc.
console.log(y)
Type casting can be implicit, where a type is automatically converted to another (e.g., when dividing an integer by another integer, resulting in a floating-point number), or explicit, as shown above.