Const in JavaScript

Like let declarations, const declarations can only be accessed within the block they were 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:

greeting.message = "say Hello instead";

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

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.

The const keyword was introduced in ES6 (2015).

• Variables defined with const cannot be Redeclared.
• Variables defined with const cannot be Reassigned.
• Variables defined with const have Block Scope.

When to use JavaScript const? As a general rule, always declare a variable with const unless you know that the value will change.