34. Explain the usage of let and const keywords in JavaScript.
In JavaScript, `let` and `const` are block-scoped keywords used to declare variables with a specific scope. These keywords were introduced in ECMAScript 6 (ES6) along with other features such as arrow functions, template literals, and destructuring assignment.
Here's how the two keywords differ:
`let`: This keyword is used to declare a variable that can be reassigned multiple times within its block scope. It has function scope by default, meaning it can be accessed from within any function defined within its block scope. Here's an example:
function myFunction() {
let x = 5;
x = 10; // x is reassigned here
}
`const`: This keyword is used to declare a constant variable that cannot be reassigned after it has been declared. It also has function scope by default, meaning it can be accessed from within any function defined within its block scope. Here's an example:
function myFunction() {
const x = 5; // x is declared as a constant variable
}
However, we cannot declare a `const` variable and reassign it later, as the reassignment would result in a TypeError:
function myFunction() {
const x = 5;
x = 10; // This will throw an error
}
In addition to function scope, we can also declare `let` and `const` variables with block scope using curly braces `{}`. Here's an example:
if (true) {
let x = 5;
}
console.log(x); // This will output 5
In this example, the variable `x` has block scope within the `if` statement and is accessible only within that block scope.
Overall, `let` and `const` are useful keywords for declaring variables with a specific scope in JavaScript. By using these keywords, we can ensure that our code is more organized and maintainable by clearly defining the scope of our variables.