Differences between var, let and const

Variables declared using var can be used within the whole function where they were initialized, but variables declared using let and const can only be inside the block where they were created.

The values of variables declared using var and let can be reassigned, but in the case of const that's not possible.

Variables declared with var can have the same name, but in the case of let and const it's not possible and you will get a Syntax Error. It's only possible to do it for the latter options if they are declared in a different scope.

Variables declared with var and let without an initial value will have undefined assigned as default. In the case of const it's not possible to declare a variable and not assign a value to it, JavaScript will return a Syntax Error.

Variables declared with var are assigned as a property of the global object, unlike in the case of let and const.

Hoisting

Variables declared using var are read by the JavaScript interpreter as if they were declared at the beginning of the function or scope. This means that the value of the variable can be referenced before it is declared in the code.

The same thing goes for let and const but in these cases they are declared without an initial value until they have an assigned value in the code. This time between the beginning of the scope and the assignment of a value to the variable is called Temporal Dead Zone. During this time a variable can't be referenced until a value is assigned to it.

This is possible because JavaScript saves in memory in the Lexical Environment the references to the declarations as undefined and then assign the value to it later when they are initialized in the code. This is only possible for the following cases:

• Class
• Function
• Const
• Let
• Var

Scope

This is an important concept because it manages the accessibility of variables in the code depending on how we handle their declaration. There are three type of scopes:

• Global
• Function & Local
• Block

Global Scope

This is the scope outside a function, all variables declared here can be used anywhere in the code.

Function & Local Scope

This is the scope inside a function, all variables declared here can only be used inside the scope of the function.

Block Scope

This is the scope inside a pair of curly braces since JavaScript ES6, all variables declared here, except for var, can only be used inside this block.