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Js Scope

📅 2026-06-19 | 📂 JavaScript

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JavaScript Scope

Scope refers to the set of variables, objects, and functions you have access to.

JavaScript Scope

In JavaScript, objects and functions are also variables.

In JavaScript, scope is the set of variables, objects, and functions you have access to.

JavaScript Function Scope: Scope is modified within functions.

JavaScript Local Scope

Variables declared inside a function are local variables and have local scope.

Local variables: Can only be accessed from within the function.

Example

// Cannot call carName variable here
function myFunction() {
    var carName = "Volvo";   // Can call carName variable here
}

Try it »

Because local variables are only accessible within the function, different functions can use variables with the same name.

Local variables are created when a function starts executing, and are automatically destroyed when the function finishes executing.

JavaScript Global Variables

Variables declared outside a function are global variables.

Global variables have global scope: All scripts and functions on a web page can use them.

Example

var carName = "Volvo";   // Can call carName variable here
function myFunction() {
    // Can call carName variable here
}

Try it »

If a variable is not declared inside a function (without the var keyword), it becomes a global variable.

In the following example, carName is inside the function but is a global variable.

Example

// Can call carName variable here
function myFunction() {
    carName = "Volvo";   // Can call carName variable here
}

Try it »

JavaScript Variable Lifecycle

JavaScript variables have a lifecycle when they are declared.

Local variables are destroyed after the function finishes executing.

Global variables are destroyed when you close the page.

Function Parameters

Function parameters act as local variables inside functions.

Global Variables in HTML

In HTML, the global scope is the window object. Therefore, the window object can call local variables declared without var inside a function.

Note: All global variables belong to the window object.

Example

// Can use window.carName here
function myFunction() {
    carName = "Volvo";
}

Try it »

Did You Know?

Note Your global variables, or functions, can overwrite variables or functions on the window object. Local variables, including the window object, can overwrite global variables and functions.

In JavaScript, local variables inside a function are generally not directly accessible from the outer scope. However, there are several ways to expose local variables within a function to the outer scope, as follows:

  • Through the global object: Inside a function, you can make a local variable globally accessible by assigning it as a property of the window object. For example, using the statement window.a = a;, you can access the value of this local variable from outside the function via window.a.
  • Defining a global variable: If you declare a variable inside a function without using keywords like var, let, or const, the variable is treated as a global variable and can be accessed outside the function. However, this practice is generally not recommended because it can lead to unintended side effects and make the code difficult to maintain.
  • Return value: You can return the value of a local variable from inside a function using a return statement, and then receive this return value outside the function. In this way, although the local variable itself is not exposed, its value can be passed to the outside through the function call.
  • Closures: The closure feature in JavaScript allows inner functions to access the local variables of outer functions. Even after the outer function has finished executing, its local variables can still be referenced by inner functions.
  • Properties and methods: Variables and functions defined in the global scope become properties and methods of the window object. Therefore, when calling them, you can omit window and use the variable or function name directly.
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