Constants and Variables Concepts

A Variable is a container which represents a value in a program. Variables can store different types of data including numeric values, single character, and text strings. The value of a variable can change all throughout a program.

A Constant is similar to a variable with one major difference - the value of a constant does not change, while the value of a variable can change all throughout a program.

There are some major points, these are:

Declaring Variables
Assigning and Re-assigning Variables
Naming Restrictions
Types of Variables

Declaring Variables

Declaring variables means to simply create a variable with a data type in front of it.

This is how, in JavaScript you can declare a variable:


							var i; // to create a variable named "i"
const j; // to create a constant variable named "j"
let k; // to create a block scope variable named "k"

JavaScript is very flexible with data types, in fact "var" can be used to declare any type of variable. Java is much stricter and you have to either declare it as an "int" or "float" etc.

Assigning Variables

When a variable is declared, the user can assign a value to it, most of the time this is needed to perform complex operations in the program.

An Example of how to assign a variable in Java - Here is the given code:


							int i; // you have declared i as an int variable 

i = 5; // int i is now given the value of 5
int j = 2; // int j is declared and assigned the value 2

System.out.println(i+j); // result is..... 5 + 2 which is... 7!

You can also re-assign different values to variables that already have values. Take a look at this example in Java:


							int i = 4;
int j = 3;

System.out.println(i+j); // result is 7

j = 5;

System.out.println(i+j); // result is now 9

Variable j was assigned the value 3. But simply using the "=" sign again we have now assigned it a new variable - 5.

The "=" sign is used to assign values to variables, whether they be strings, intgers, floats, constants, functions, arrays and objects. This makes many programming languages flexible to use when it comes to assigning values, as you can usually change values very quickly.

Types of Variables

In Javascript there are different ways to declare variables (var, let, const). let and const were introduced with in the ES6 update in 2015.

This is an example of how to declare a block scope local variable (let) in JS:


							let i = 5;
for(i; i<9; i++){
	
	console.log(i);

}

// will print 5,6,7,8

"let" allows you to declare variables that are limitd in scope to the block, statement, or expression on which it is used. This is unlike the "var" keyword, which defines a variable globally, or locally to an entire function regardless of block type. The "const" keyword is different however:


							const num = 23;

try{
	num = 22;
} catch (err){
	console.log(err);
}

console.log(num);

//If there was no try, catch statement to catch errors then num will cause an error.
//This is because constants (const) cannot be changed as they are constant variables.

//The expected output is 23 as the error is caught.

Constants are block scoped, much like variables defined using the "let" statement. The value of constant cannot change through re-assignment however.

This is to create a difference between the different types of variable statements to help organise your code better as certain statements help with scope size and limitations whilst others are constant and others like "var" are just general.

These are the major points of Variables and Constants.