JavaScript global variable
Like other programming languages, JavaScript also has local and global variables . In JavaScript, objects and functions are also variables. The scope specifies from where you can access a variable and whether you have access to the variable in that context. JavaScript variables has two scopes : global and local. A variable that is declared outside a function definition is called global variable, and its value is accessible and modifiable throughout your program. A variable that is declared inside a function definition is called local variable . It is created and destroyed every time the function is executed, and it cannot be accessed by any code outside the function.JavaScript Local Variables
A variable that is declared within a JavaScript function has become local to the function and it can only be accessed within the function.
example
function doSomething() {
var name = "John";
//variable name can use only inside the doSomething()
}
Local variables are created when a function starts, and deleted when the function is completed.
JavaScript Global Variables
A variable that is declared outside of a function definition is called a Global Variable and it has global scope, all scripts and functions on a web page can access it and modify throughout the program. example
var total=10;
function addValue(){
total = total +100;
}
function dispValue(){
alert("Total is :" + total);
}
addValue();
dispValue();
Here a Global Variable declared as total. Then add values to the total inside a local function addValue(). Next, dispValue() function display the value of total. Here you can see the global variable value modify inside a local function and the new value is displayed in another local function. It happens because of the scope of Global Variable.
Automatically Global
If a variable is assigned a value without first being declared with the var keyword, it is automatically added to the global context and it is thus a global variable: example
function addValue(){
total = 100;
}
function dispValue(){
alert("Total is :" + total);
}
addValue();
dispValue();
Here you can see, the variable total is not declared. Inside the addValue() function variable total assigned a value. So, its scope is global because its not declared.
Note: Accessing a global variable is required a lot of times. If a variable is going to be used throughout the program it is important that you declare the variable in global scope . But, because JavaScript allows re-declaration of same variables, it can easily become a problem if a function is using a variable with same name as global variable in its scope.
Related Topics