Javascript : #2 Variables and Data types

 

What is a Variable?!

A variable is a container with a label (name) where you store a certain value ( a piece of data ) to use it in your code.

Declaring variables and assigning values to them

To declare a variable is to give it a name, you have to use one of these keywords:

• var : most common key to declare variables.
• let : only visible within the block where it's declared.
• const : used for constant values and cannot be changed , it will cause an error if you tried to do so.

 To declare a variable, use one of the keywords followed by the variable name (which has to be meaningful)

var myVariable ;
let myName ;
const pi ;




Variable naming conventions

To name a variable in javascript , you should follow some rules:

• javascript is case sensitive which means that the letters' case matters (VARIABLE isn't the same as Variable and also not as variable).
• you have to use camelCase (example: myName, schoolYear, etc.)
• You can add numbers but not at the beginning (whatever , who will name a variable 2myVarName, this won't satisfy the rule n.o. 2)
• you cannot use hyphens " - " or spaces but you can use underscores " _ " instead (note : underscores can be used as the first character in a variable name).
• You cannot use any of the reserved keywords (like : var, let, const, if, while, for and so on) . don't worry if you don't know a lot of keywords , if you tried to use one it will be highlighted as a keyword and results in an error.

Assigning values to variables

To assign a value to a variable, use the assignment operator  " = " (equal sign).

// Declaring variables
var myVariable ; 
let myName ;
const pi ;

// assigning values to variables
myVariable = 25; 
myName= "Rawan";
pi = 3.14;

Note : you can declare variables and assign values to them on the same line.

// Declaring a variable and assigning a value to it on one line
var myAge = 15; 




Datatypes

In Javascript there are a lot of data types, we will discuss the most important and basic ones.

Main Datatypes :

• Numbers : they can be integer numbers or floats( numbers with decimal points).
• Strings : any series of characters included between quotes (double quotes " " or single quotes ' ' ).
• Boolean value : has one of the two values true or false .
• null : means nothing.
• undefined : something that hasn't been defined.
• Arrays : can store more than one piece of data.
• Objects : used to store key-value pairs( each property associated with its own value).

Getting output in Javascript 

You can output values to the console using console.log() , inside the parenthesis put a variable name or any piece of data to be shown in the console.

console.log("Hello World !"); // output : Hello World !
var myScore = 320 ;
console.log(myScore); // output : 320

Notes , Again...

1 . Everything greyed out (not highlighted) after " // " is called a comment , you write them to explain to yourself and others what your code does , you write them using :  

• // : for inline comments
• /*  */ : for multi-line comments

// I am an inline comment

/* I am a 
multi-line
comment */

2 . In Javascript we put a semicolon "; " after the end of each line, you don't have to do it all the time, but it's a good practice to do so.

3 . Variables that are declared but not assigned to store any values are called " Uninitialized variables " and have a default value of undefined .