Local vs Global variables in JavaScript

In some places globalization is indeed wrong, here's one: Coding.

In coding, variables are used to store values that can be accessed and manipulated by the program. In JavaScript (JS), variables can be either local or global, and it is essential to understand the difference between them and their usage. Let's discuss local and global variables, their differences, and how to use them in JS programming.

Local Variables

Local variables are variables that are declared within a function or block of code. These variables are only accessible within the block in which they are defined. When a function is called, it creates a new scope, which is a temporary environment for the variables used in the function.

Let's look at an example of a local variable in JS:

const calculateArea = (radius) => { 
    const pi = 3.14159; // local variable 
    let area = pi * radius * radius; 
    return area; 
}

In the above code, pi is a local variable, which is only accessible within the calculateArea function. When the function is called, it creates a new scope, which contains the local variable pi. This variable is not accessible outside of the function, and it is destroyed when the function finishes executing.

Global Variables

Global variables are variables that are declared outside of a function or block of code. These variables can be accessed and manipulated by any part of the program, including functions. Global variables are typically used for values that need to be shared between multiple functions or across the entire program.

Let's look at an example of a global variable in JS:

const pi = 3.14159; // global variable
const calculateArea = (radius) => { 
    let area = pi * radius * radius; 
    return area; 
}

In the above code, pi is a global variable, which is accessible to any part of the program. The calculateArea function can access this variable and use it in its calculation. Global variables are useful when we need to store values that are used across different parts of the program.

Differences between Local and Global Variables

The main difference between local and global variables is their scope. Local variables have a limited scope, which is limited to the block or function in which they are defined. In contrast, global variables have a wider scope, which is accessible to any part of the program.

Another difference between local and global variables is their lifetime. Local variables are created when a function is called and destroyed when the function finishes executing. In contrast, global variables are created when the program starts and destroyed when the program ends.

Best Practices for Using Local and Global Variables

When using variables in JS, it is important to follow best practices to avoid errors and make your code more readable and maintainable. It is generally a good practice to use local variables when possible, as they have a smaller scope and are less likely to cause naming conflicts with other variables. Local variables are also more efficient since they are created and destroyed quickly.

Here are some best practices for using local and global variables: Use local variables when possible.

Use descriptive names:

// Good: Descriptive names accurately reflect the purpose of the variables 
let firstName = "Albus"; 
let lastName = "Veritas";

// Bad: Nondescriptive names make it harder to understand what the variables are used for 
let x = "Albus"; 
let y = "Veritas";

Limit the scope of variables:

// Good: Limit the scope of variables to only what is necessary 
const calculateSum = (a, b) => a + b

// Bad: Global variables can cause unintended consequences and make code harder to maintain 
let sum = 0;
const addToSum = (a) => { 
    sum += a; 
}

Use let and const instead of var:

// Good: Use let and const to declare variables with block-level or constant scope 
for (let i = 0; i < 5; i++) { 
    console.log(i); 
}
const MAX_ITEMS = 10;

// Bad: Use of var can lead to unexpected behavior 
for (var i = 0; i < 5; i++) { 
    console.log(i); 
}

// The value of i is accessible outside of the for loop because var has function-level scope 
console.log(i); // 5

Declare variables before use:

// Good: Declare variables before use to avoid the "temporal dead zone" 
let x = 10; 
let y = 20;

// Bad: Variables declared with let or const cannot be accessed until they are declared 
console.log(z); // ReferenceError: z is not defined 
let z = 30;

Pass variables as arguments:

// Good: Pass variables as arguments to maintain the encapsulation and separation of concerns of each function 
const multiply = (a, b) => a * b; 
let result = multiply(2, 3);

// Bad: Global variables can cause unintended consequences and make code harder to maintain 
let a = 2; 
let b = 3;
const multiply = () => a * b; 
let result = multiply();

Conclusion

Understanding the difference between local and global variables is an essential aspect of programming in JavaScript. Local variables are defined within a function and are only accessible within that function, while global variables are defined outside of any function and are accessible from anywhere in the program.

Using local variables whenever possible and limiting the use of global variables can help you write more efficient, maintainable, and scalable code in JavaScript.

Dhawal Codes