Var, Let, & Const – What's the Difference? | simplest explanation | LET | VAR | CONST in Javascript
Var, Let, & Const – What's the Difference? | simplest explanation | LET | VAR | CONST in Javascript
A lot of shiny new features came out with ES2015 (ES6). And now, since it's 2020, it's assumed that a lot of JavaScript developers have become familiar with and have started using these features.
While this assumption might be partially true, it's still possible that some of these features remain a mystery to some devs.
One of the features that came with ES6 is the addition of let and const, which can be used for variable declaration. The question is, what makes them different from good ol' var which we've been using? If you are still not clear about this, then this article is for you.
In this article, we'll discuss var, let and const with respect to their scope, use, and hoisting. As you read, take note of the differences between them that I'll point out.
Var
Before the advent of ES6, var declarations ruled. There are issues associated with variables declared with var, though. That is why it was necessary for new ways to declare variables to emerge. First, let's get to understand var more before we discuss those issues.
Scope of var
Scope essentially means where these variables are available for use. var declarations are globally scoped or function/locally scoped.
The scope is global when a var variable is declared outside a function. This means that any variable that is declared with var outside a function block is available for use in the whole window.
var is function scoped when it is declared within a function. This means that it is available and can be accessed only within that function.
Let
let is now preferred for variable declaration. It's no surprise as it comes as an improvement to var declarations. It also solves the problem with var that we just covered. Let's consider why this is so.
let can be updated but not re-declared.
Just like var, a variable declared with let can be updated within its scope. Unlike var, a let variable cannot be re-declared within its scope. So while this will work:
Const
Variables declared with the const maintain constant values. const declarations share some similarities with let declarations.
const declarations are block scoped
Like let declarations, const declarations can only be accessed within the block they were declared.
const cannot be updated or re-declared
This means that the value of a variable declared with const remains the same within its scope. It cannot be updated or re-declared. So if we declare a variable with const, we can neither do this:
let is block scoped
A block is a chunk of code bounded by {}. A block lives in curly braces. Anything within curly braces is a block.
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