const let var - The difference

In JavaScript, there are three keywords used to declare variables: 'var', 'let', and 'const'. Each of these keywords has its own characteristics and best practices for usage. Let's explore their differences and when to use them effectively.

var Keyword:

The 'var' keyword is the oldest way to declare variables in JavaScript.

• Scope: Variables declared with 'var' have function scope or global scope. They are accessible throughout the entire function or globally if declared outside any function.
• Hoisting: 'var' variables are hoisted to the top of their scope, allowing them to be accessed before their declaration.
• Reassignment: 'var' variables can be reassigned and updated.

Example:

console.log(a); // Output: undefined
var a = 10;
console.log(a); // Output: 10

let Keyword:

The 'let' keyword was introduced in ECMAScript 6 (ES6) to overcome some of the limitations of 'var'.

• Scope: Variables declared with 'let' have block scope, which means they are only accessible within the block where they are defined (e.g., inside loops or conditionals).
• Hoisting: 'let' variables are not hoisted, so they cannot be accessed before their declaration.
• Reassignment: 'let' variables can be reassigned and updated within their scope.

Example.

console.log(b); // Output: ReferenceError: b is not defined
let b = 9;
console.log(b); // Output: 9

const Keyword:

The 'const' keyword is used to declare constants, which are variables that cannot be reassigned once they are assigned a value.

• Scope: Variables declared with 'const' also have block scope.
• Hoisting: 'const' variables are not hoisted and need to be initialized during declaration.
• Reassignment: 'const' variables cannot be reassigned after initialization, making them read-only.

Example:

const c = 10;
console.log(c); // Output: 10

c = 9; // Error: TypeError: Assignment to constant variable.

Best Practices:

• Use 'const' for values that should not change (e.g., configuration settings, fixed values).
• Use 'let' for variables that need to be reassigned or updated within their scope.
• Minimize the use of 'var' since it has function scope and can lead to unintended consequences.

By understanding the differences and best practices of 'var', 'let', and 'const', you can write more reliable and maintainable JavaScript code. Choose the appropriate keyword based on the desired scope and mutability of your variables.