What is local, script and global scope in Node.js ?

Node.js is an open-source and cross-platform runtime environment for executing JavaScript code outside a browser. You need to remember that NodeJS is not a framework and it’s not a programming language. Most people are confused and understand it’s a framework or a programming language. We often use Node.js for building back-end services like APIs like Web App or Mobile App. It’s used in production by large companies such as Paypal, Uber, Netflix, Walmart, and so on.

Scope of a Variable: The word scope means the extent to which we can use something. In programming variable scope means the extent to which we can use a variable within a program.

In JavaScript there are mainly 3 types of scopes:

1. Local scope 
2. Global scope
3. Script scope

Local Scope:

• The Node.js application starts with the local scope enabled. Local variables are defined by the functions. Variables having a local scope restricts their usage to the function in which they are defined.
• It’s also known as Default Scope.

Example 1: An example of defining variables under local scope-

Javascript

var d = 'Tom';
function zeta() {
    var d;
    if (d === undefined) {
        d = 'Jerry';
    }
    console.log(d);
}
zeta();

Output:

Jerry

Explanation: The zeta function is used to declare a new variable d which possesses a local scope. In JavaScript, the variables are initiated at the top of the current scope no matter where it is included in the code while we use var to declare them.

Example 2: Here is another example of declaring variables under local scope –

Javascript

var locales = {
    europe: function () {
        // The Europe continent's local scope
        var myFriend = "Monique";

        // France country's local scope
        var france = function () {

            // The Paris city's local scope
            var paris = function () {

                // output: Monique
                console.log(myFriend);
            };

            paris();
        };

        france();
    }
};

locales.europe();

Output:

Monique

Explanation: The myFriend variable was declared in the outer scope of the France function and is thus accessible from the Paris function in this case. MyFriend is not declared if the myFriend variable and the console statement are switched.

Global Scope:

•  When you’re in the global scope var something of a browser will define a global variable because the global scope is the highest scope in browsers where it is not the same case in Node. Inside a Node module, a variable will be limited to that module only; and global scope will not be the top scope anymore.

Example 1: An example of declaring variables under Global scope-

Javascript

// Global Scope

const name = "Monique";

function sayHi() {
    console.log(`Hi ${name}`);
}

sayHi();
// Hi Monique

Output:

Hi Monique

Explanation: We can see from this simple example given above that the variable name is global. It is available throughout the entire application and is defined in the global scope.

Example 2: Here is another example illustrating global scope –

Javascript

//Global Variable in JavaScript
var scope = "Global"; //Global Variable
console.log(scope); //Global
function access() {
    console.log(scope); // Accessing a global variable
}

access(); //Global

Output:

Global
Global

Explanation: Given above is a simple program in JavaScript in which we have declared the variable Global using the keyword var under the global scope. In Global scope variables we can use the three keywords: let, const, and var. 

Script:

• The Script scope is actually a place to keep the variables that are shared and strictly not exposed to the Global scope.
• Each child in a script or script module shares the Script scope.
•  Essentially, they are a series of terminal commands or terminal commands themselves that assist in automating repetitive chores. NPM scripts as a series of terminal commands can be summed up. Scripts for a project are kept in the package’s section.

Javascript

let GLOBAL_DATA = { value: 1 };
console.log(GLOBAL_DATA);

Output:

{ value: 1 }

Thanks for reading. Comment below for any improvement or suggestion.