In this lesson, you'll learn about scope in JavaScript, a fundamental concept that determines the accessibility of variables, functions, and objects in different parts of your code.

What is Scope and Why Do We Need It?

Scope allows us to know where we have access to our variables. It shows us the accessibility of variables, functions, and objects in some particular part of the code.

Why Limit Visibility?

• Security: Limiting the visibility of variables provides a level of security to your code.
• Efficiency: It helps improve efficiency, track bugs, and reduce them.
• Naming Conflicts: It solves the problem of naming variables.

Types of Scope

• Global Scope
• Local Scope
• Block Scope ( only with and )

Variables defined inside a function are in local scope , while variables defined outside of a function are in the global scope . Each function, when invoked, creates a new scope.

There are rules about how scope works, but usually, you can search for the closest { } curly braces around where you define the variable. That “block” of code is its scope .

Global Scope

When you start writing in a JavaScript document, you're already in the global scope.

const name = 'Adrian';

Variables written inside the global scope can be accessed and altered in any other scope.

const logName = () => {
    console.log(name);
}

logName();

Advantages of Using Global Variables

• Accessibility: You can access the global variable from all the functions or modules in a program.
• Consistency: Ideally used for storing "constants" as it helps you keep the consistency.
• Data Sharing: Useful when multiple functions are accessing the same data.

Disadvantages of Using Global Variables

• Memory Usage: Too many variables declared as global remain in memory until program execution is completed, which can cause out-of-memory issues.
• Unpredictable Results: Data can be modified by any function, leading to unpredictable results in multi-tasking environments.
• Refactoring Challenges: If global variables are discontinued due to code refactoring, you will need to change all the modules where they are called.

Local Scope

Variables defined inside a function are in the local scope.

// Global Scope

const someFunction = () => {
    // Local Scope #1

    const anotherFunction = () => {
        // Local Scope #2
    }
}

Advantages of Using Local Variables

• Data Integrity: The use of local variables offers a guarantee that the values of variables will remain intact while the task is running.
• Naming Flexibility: You can give local variables the same name in different functions because they are only recognized by the function they are declared in.
• Memory Efficiency: Local variables are deleted as soon as any function is over, releasing the memory space they occupy.

Disadvantages of Using Local Variables

• Limited Scope: They have a very limited scope. If you need to use a variable in a parent scope, declare it there.

Block Scope

Block statements like , , and loops don't create a new scope with .

However, variables defined with or have block scope, meaning they are only available inside the block of code where they are created.

if (true) {
    var name = 'Adrian'; // global scope
    let likes = 'Coding'; // block scope
    const skills = 'JavaScript and PHP'; // block scope
}

console.log(name); // logs 'Adrian'
console.log(likes); // Uncaught ReferenceError: likes is not defined
console.log(skills); // Uncaught ReferenceError: skills is not defined

What is More Useful?

Both and are important. However, a large number of global variables can occupy a lot of memory and make changes difficult to track.

It's advisable to avoid declaring unnecessary global variables and always declare variables in the scope where you intend to use them .

Key Differences

Understanding is crucial for writing efficient and error-free JavaScript code. It helps you manage variable accessibility and avoid potential conflicts.

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