Variable

A variable is a “named storage” for data. We can use variables to store goodies, visitors, and other data. In JavaScript, there are different ways of creating variables which includes var, let, and const. Here, we will talk about the scope and difference between these three ways.

Var

The var statement declares a function-scoped or globally-scoped variable, optionally initializing it to a value

var declarations, wherever they occur, are processed before any code is executed. This is called hoisting and is discussed further below.

The scope of a variable declared with var is its current execution context and closures thereof, which is either the enclosing function and functions declared within it, or, for variables declared outside any function, global. Duplicate variable declarations using var will not trigger an error, even in strict mode, and the variable will not lose its value, unless another assignment is performed.

Here we created and called a function, printName, that will print the value of the name var, Madison. You’ll see this printed in your console.

Because the var was created outside of the function, it is globally scoped. This means that it is available anywhere in your code, including inside of any function. This is why our function, printName, has access to the name var.

Let’s create a variable that is function-scoped. This means that the variable is only accessible inside the function it was created in. This next example will be very similar to the code above, but with a different placement of the variable.

let

The let declaration declares a block-scoped local variable, optionally initializing it to a value.

let allows you to declare variables that are limited to the scope of a block statement, or expression on which it is used, unlike the var keyword, which declares a variable globally, or locally to an entire function regardless of block scope.

However, let differs from const in a fundament way. Variables declared with the let keyword can be redeclared, while variables created with the const keyword cannot. Going over an example we have:

By using the let keyword, we’ve updated our variable to point to the value of true as we wanted to. Sometimes in programming we’ll want to update our variable depending on certain data that we receive. We can use let to do this.

Const

The const keyword was also introduced in the ES6(ES2015) version to create constants. For example,

Once a constant is initialized, we cannot change its value.

A constant is a type of variable whose value cannot be changed.

Also, you cannot declare a constant without initializing it. For example,

Note: If you are sure that the value of a variable won't change throughout the program, it's recommended to use const.

Difference between var, let and const

Var

• The scope of a var variable is functional scope

• It can be updated and redeclared into the scope

• It can be declared without initialization

• It can be accessed without initialization as it's default value is "undefined".

• Hoisting done, with initializing as "default value".

Let

• The scope of a let variable is block scope

• It can be updated but cannot be redeclared into the scope

• It can be declared without initialization

• It cannot be accessed without initialization otherwise it will give "reference error".

• Hoisting is done but not initialized.

Const

• The scope of a const variable is block scope

• It cannot be updated or redeclared into the scope

• It cannot be declared without initialization

• It cannot be accessed without initialization, as it cannot be declared without initialization.

• Hoisting is done but not initialized.