Javascript Variables
In JavaScript, a variable is a named storage location that holds a value. Variables are used to store and manipulate data in a program. In JavaScript, there are three keywords used to declare variables:
var
: This keyword is used to declare a variable in the global scope or function scope. A variable declared usingvar
can be re-declared and re-assigned within the same scope.let
: This keyword is used to declare a variable in the block scope. A variable declared usinglet
can be re-assigned but cannot be re-declared within the same scope.const
: This keyword is used to declare a constant variable that cannot be re-assigned or re-declared within the same scope. A variable declared usingconst
must be initialized when it is declared.
Here are some examples of how to declare and use variables in JavaScript:
javascriptCopy code// Declaring a variable using var
var x = 5;
var y = 10;
var z = x + y;
// Declaring a variable using let
let a = 2;
let b = 3;
let c = a * b;
// Declaring a constant variable using const
const PI = 3.14159;
const radius = 5;
const circumference = 2 * PI * radius;
In the above examples, variables are declared using var
, let
, and const
keywords. These variables are assigned values and used in expressions to perform calculations. The variables can be accessed and modified throughout the program, depending on the scope in which they are declared.
It's important to choose the right type of variable declaration based on the requirements of the program. var
is used for backward compatibility and is generally not recommended for modern code. let
is used for variables that may change their value over time. const
is used for values that are constant and should not be changed.
In JavaScript, there are some rules that need to be followed when declaring variables:
Variable names must start with a letter, underscore (
_
), or dollar sign ($
). They cannot start with a number.Variable names can only contain letters, numbers, underscores, or dollar signs.
Variable names are case-sensitive, meaning that
myVariable
is different fromMyVariable
.Avoid using reserved words as variable names. JavaScript has a number of reserved words that have a special meaning in the language, and using them as variable names can cause unexpected behavior. Some examples of reserved words include
if
,else
,for
,while
,true
,false
, andnull
.Use camelCase notation for multi-word variable names. For example,
firstName
is a better variable name thanfirst_name
.Always declare your variables with
var
,let
, orconst
. Failing to declare your variables can create global variables, which can lead to naming conflicts and other issues.
Here are some examples of correct and incorrect variable declarations in JavaScript:
javascriptCopy code// Correct variable declaration
let myVariable = "Hello, world!";
// Incorrect variable declaration (starts with a number)
let 1stPlace = "First place";
// Incorrect variable declaration (contains a hyphen)
let my-variable = "Invalid variable name";
// Incorrect variable declaration (uses a reserved word)
let if = "Invalid variable name";
// Correct variable declaration (uses camelCase notation)
let firstName = "John";
let lastName = "Doe";
By following these rules, you can create meaningful and reliable variable names that will make your code easier to understand and maintain.