Difference between const and let

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 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.

So a variable declared in a block with let is only available for use within that block. Let me explain this with an example:

Example:

let greeting = "say Hi";
let times = 4;

if (times > 3) {
    let hello = "say Hello instead";
    console.log(hello);// "say Hello instead"
}
console.log(hello) // hello is not defined

We see that using hello outside its block (the curly braces where it was defined) returns an error. This is because let variables are block scoped .

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:

let greeting = "say Hi";
greeting = "say Hello instead";

this will return an error:

let greeting = "say Hi";
let greeting = "say Hello instead"; // error: Identifier 'greeting' has already been declared

However, if the same variable is defined in different scopes, there will be no error:

let greeting = "say Hi";
if (true) {
    let greeting = "say Hello instead";
    console.log(greeting); // "say Hello instead"
}
console.log(greeting); // "say Hi"

Why is there no error? This is because both instances are treated as different variables since they have different scopes.

This fact makes let a better choice than var. When using let, you don't have to bother if you have used a name for a variable before as a variable exists only within its scope.

Also, since a variable cannot be declared more than once within a scope, then the problem discussed earlier that occurs with var does not happen.

Hoisting of let

Just like var, let declarations are hoisted to the top. Unlike var which is initialized as undefined, the let keyword is not initialized. So if you try to use a let variable before declaration, you'll get a Reference Error.

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:

const greeting = "say Hi";
greeting = "say Hello instead";// error: Assignment to constant variable. 

nor this:

const greeting = "say Hi";
const greeting = "say Hello instead";// error: Identifier 'greeting' has already been declared

Every const declaration, therefore, must be initialized at the time of declaration.

This behavior is somehow different when it comes to objects declared with const. While a const object cannot be updated, the properties of this objects can be updated. Therefore, if we declare a const object as this:

const greeting = {
    message: "say Hi",
    times: 4
}

while we cannot do this:

greeting = {
    words: "Hello",
    number: "five"
} // error:  Assignment to constant variable.

we can do this:

```js
greeting.message = "say Hello instead";

This will update the value of greeting.message without returning errors.

Hoisting of const

Just like let, const declarations are hoisted to the top but are not initialized.

So just in case you missed the differences, here they are:

  • var declarations are globally scoped or function scoped while let and const are block scoped.
  • var variables can be updated and re-declared within its scope; let variables can be updated but not re-declared; const variables can neither be updated nor re-declared.
  • They are all hoisted to the top of their scope. But while var variables are initialized with undefined, let and const variables are not initialized.
  • While var and let can be declared without being initialized, const must be initialized during declaration.
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