Javascript : Variables and Datatypes

 

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 u…


This content originally appeared on DEV Community and was authored by Rawan Amr Abdulsattar

 

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 .


This content originally appeared on DEV Community and was authored by Rawan Amr Abdulsattar


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