When to use var, let, vs const in JavaScript?

In JavaScript, there are three keywords used to declare variables: var, let, and const. Each of these has its own specific use case, depending on the context in which it's used. Here's a brief overview of when to use each one:

The "var" keyword is the oldest way of declaring variables in JavaScript. It declares a variable that is function-scoped, which means that it can be accessed within the function in which it was declared. If you declare a variable using "var" outside of a function, it becomes a global variable, which can be accessed from anywhere in your code.

In general, it's best to avoid using "var" these days, and instead use "let" and "const" (depending on the situation) because "var" has some quirks that can lead to unexpected behavior in your code.

The "let" keyword is used to declare variables that are block-scoped, which means they can only be accessed within the block in which they were declared (e.g., within a loop or an if statement). "let" variables can be reassigned a new value, which makes them useful when you need to change the value of a variable over time.

Use "let" when you need to declare a variable that will be reassigned or if you need block-scoping.

The "const" keyword is used to declare variables that are also block-scoped, but unlike "let", they cannot be reassigned a new value. Once you declare a "const" variable, its value is fixed and cannot be changed. This makes "const" variables useful when you have a value that should never change, such as a constant like pi or e.

Use "const" when you want to declare a variable that should never be changed.

In general, it's a good practice to use "let" by default and only use "const" when you have a value that should never change. This helps make your code more predictable and easier to reason about.