Understand the differences in scope and usage for var, let, and const.
In modern JavaScript (ES6 and beyond), understanding the distinction between `var`, `let`, and `const` is fundamental for managing state and avoiding common bugs. `var` was the original way to declare variables. It is function-scoped, meaning it is accessible anywhere within the function it's declared in, regardless of block constructs like `if` statements or `for` loops. A key characteristic of `var` is hoisting, where the declaration is moved to the top of its scope, but the initialization is not. This can lead to confusing behavior where a variable can be used before it's declared. To address these issues, ES6 introduced `let` and `const`. Both are block-scoped, meaning they are only accessible within the block (`{...}`) they are defined in. This provides a more predictable and granular control over a variable's lifecycle. `let` allows you to declare variables that can be reassigned later. It is the go-to choice for variables that need to change, such as loop counters or values that get updated based on user input. `const`, on the other hand, is for declaring constants—variables whose value should not be reassigned after their initial declaration. It's important to note that for objects and arrays declared with `const`, the reference is constant, but the contents of the object or array can still be modified. As a best practice, you should default to using `const` and only use `let` when you know a variable's value needs to change.