Javascript shorts - const, let and var

Javascript has three ways to declare variable. In this article I will make a quick summary and briefly describe all the differences.

const numberA = 5;
let numberB = 5;
var numberC = 5;

Scope

Depending where var is declared it can be globally scoped, when declared outside of function, or function scoped otherwise. A function scope means that it can be accessed within the function. A variable declared in a block with let or const is only available for use within that block.

Can it be updated?

 var user = 'Tommy';
 user = 'John';

 let user2 = 'Tommy';
 user2 = 'John';

 const user3 = 'Tommy';
 user3 = 'John'; // <-- Error

A var or let declared variable can be updated. A const declared variable cannot be updated when it is a primitive value. The property of const object or value inside the const array can be changed but reference must stay the same.

const obj = {name: 'Tommy'};
obj.age = 30 // <-- OK
obj = {id: 1} // <-- Error

Can it be re-declared?

 var user = 'Tommy';
 var user = 'John';

 let user2 = 'Tommy';
 let user2 = 'John'; // <-- Error

 const user3 = 'Tommy';
 const user3 = 'John'; // <-- Error

Once a let or const variable is declared it cannot be declared again within the same scope. A var declared variable can be re-declared.

Hoisting

The var variables are hoisted to the top of their scope and initialized with a value of undefined. It can be declared and accessed without initialization. The let declarations are hoisted to the top. Unlike var, the let keyword is not initialized, it can be declared without initialization but if you try to use it before you set the value, you'll get a Reference Error. The const variables are hoisted to the top and must be initialized during declaration.

Why you should avoid using var?

Since var declared variable can be later re-declared and it is not block scoped it can be very easy to make a simple mistake. Just look at the code below:

 var user = 'Tommy';

 if (true) {
    var user = 'John';
 }

 console.log(user) // 'John'

Even if you declared a variable inside a block you may be aware that this variable has already been declared before. Changes inside code block affects the code outside of it and this may give unexpected results and lots of bugs later during development. This is why you should always use const and let instead of var.