Mastering JavaScript Scope | JavaScript Scope Decoded | Exploring the Depths of JavaScript Scope | Clearing the Confusion for Developers

Welcome to our guide on JavaScript scope — a concept essential for understanding how variables work in JavaScript code. Whether you’re just starting out or looking to refresh your knowledge, understanding scope is like mastering the map of a new land — it helps you navigate through your code confidently.

In this guide, we’ll explain scope in simple terms. Think of scope as the set of rules that determines where in your code you can find and use different variables. It’s like the boundaries that define where things are valid and accessible.

Before we dive into the specifics, let’s understand why scope matters. Imagine you’re in a kitchen trying to find ingredients. Each cupboard represents a different part of your code, and each ingredient is like a variable. Scope tells you which cupboards (or parts of your code) you can look in to find specific ingredients (variables).

Throughout this guide, we’ll use easy-to-follow examples and clear explanations to help you grasp the concept of scope. By the end, you’ll be able to confidently navigate through your JavaScript code, knowing exactly where to find and use your variables.

So, whether you’re new to JavaScript or looking to strengthen your understanding, let’s embark on this journey together to demystify JavaScript scope.

Let’s get started!

Scope determines the accessibility (visibility) of variables.

JavaScript variables have 3 types of scope:

Block scope

Function scope

Global scope

Block Scope

Before ES6 (2015), JavaScript variables had only Global Scope and Function Scope.

ES6 introduced two important new JavaScript keywords: let and const.

These two keywords provide Block Scope in JavaScript.

Variables declared inside a { } block cannot be accessed from outside the block:

Example

{
  let x = 2;
}
// x can NOT be used here

Variables declared with the var keyword can NOT have block scope.

Variables declared inside a { } block can be accessed from outside the block.

{
  var x = 2;
}
// x CAN be used here

Local Scope

Variables declared within a JavaScript function, are LOCAL to the function:

Example

myFunction();

function myFunction() {
  let carName = "Volvo";
  console.log(typeof carName + " " + carName);
}
console.log(typeof carName);

Local variables have Function Scope:

They can only be accessed from within the function.

Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Local variables are created when a function starts, and deleted when the function is completed.

Function Scope

JavaScript has function scope: Each function creates a new scope.

Variables defined inside a function are not accessible (visible) from outside the function.

Variables declared with var, let and const are quite similar when declared inside a function.

They all have Function Scope:

function myFunction() {
  var carName = "Volvo";   // Function Scope
}
function myFunction() {
  let carName = "Volvo";   // Function Scope
}

function myFunction() {
  const carName = "Volvo";   // Function Scope
}

Global JavaScript Variables

A variable declared outside a function, becomes GLOBAL.

let carName = "Volvo";
myFunction();

function myFunction() {
  console.log("I can display " + carName);
}

A global variable has Global Scope:

All scripts and functions on a web page can access it.

Global Scope

Variables declared Globally (outside any function) have Global Scope.

Global variables can be accessed from anywhere in a JavaScript program.

Variables declared with var, let and const are quite similar when declared outside a block.

They all have Global Scope:

var x = 2;       // Global scope
let x = 2;       // Global scope
const x = 2;       // Global scope

JavaScript Variables

In JavaScript, objects and functions are also variables.

Scope determines the accessibility of variables, objects, and functions from different parts of the code.

Automatically Global

If you assign a value to a variable that has not been declared, it will automatically become a GLOBAL variable.

This code example will declare a global variable carName, even if the value is assigned inside a function.

myFunction();
// code here can use carName as a global variable
console.log("I can display " + carName);

function myFunction() {
  carName = "Volvo";
}

Strict Mode

All modern browsers support running JavaScript in “Strict Mode”.

You will learn more about how to use strict mode in a later chapter of this tutorial.

In “Strict Mode”, undeclared variables are not automatically global.

Global Variables in HTML

With JavaScript, the global scope is the JavaScript environment.

In HTML, the global scope is the window object.

Global variables defined with the var keyword belong to the window object:

Example

var carName = "Volvo";

// code here can use window.carName
console.log("I can display " + window.carName);

Global variables defined with the let keyword do not belong to the window object:

Example

let carName = "Volvo";

// code here can not use window.carName
console.log("I can not display " + window.carName);

Warning

Do NOT create global variables unless you intend to.

Your global variables (or functions) can overwrite window variables (or functions).

Any function, including the window object, can overwrite your global variables and functions.

The Lifetime of JavaScript Variables

The lifetime of a JavaScript variable starts when it is declared.

Function (local) variables are deleted when the function is completed.

In a web browser, global variables are deleted when you close the browser window (or tab).

Function Arguments

Function arguments (parameters) work as local variables inside functions.

Conclusion

And that’s a wrap! You’ve just taken a journey through the world of JavaScript scope, and hopefully, it feels a bit less mysterious now.

Think of scope like a set of rules that determine where you can find and use your variables in your code. It’s like having different rooms in a house, each with its own set of stuff you can access.

Understanding scope is key to writing code that behaves predictably and efficiently. By grasping these concepts, you’re better equipped to build awesome things with JavaScript.

So, as you continue on your coding adventure, keep practicing with scope. Try out different scenarios, experiment with code, and don’t be afraid to make mistakes — that’s how we learn!

Thanks for joining us on this journey. We hope you feel more confident about scope and ready to tackle even more exciting challenges in JavaScript. Happy coding!

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