Welcome to the javascript learning course

Lets start today's topic - How many ways are there to declare variable in javascript.

var
let
const

Lets explore all ways:

Let's start with var. Declare a variable by this syntax


copy
var a

Assign a value to the varibale .


copy
var a = 1

Reassign a value to the variable .


copy
a = 3

Lets see what happens when we declare a variable and print it's value to console.


copy
var a
console.log(a)  // undefined

undefined is printed on console because we didn't assign any value to the variable 'var a'.

Let's see what happens when we assign a value to the variable.


copy
var a = 1
console.log(a)  // 1
a = 3
console.log(a)  // 3

We can see that 1 is printed and when we reassign the value 3 to a, 3 is printed.

var with same name can be redeclared again it will replace the previously defined value.


copy
var a
console.log(a)  // undefined
var a = 1;
console.log(a)  // 1
var a = 3;
console.log(a)  // 3

Next is let way to declare a variable.


copy
let b
console.log(b)  // undefined

When value is not assigned to let is print's undefined.


copy
let b = 1
console.log(b)  // 1
b = 4
console.log(b)  // 4

Value to b can be reassigned to let.

let with the same name cannot be redeclared again it throw's syntax error.


copy
let b
console.log(b)  // undefined
let b = 1
console.log(b)  // Uncaught SyntaxError: Identifier 'b'
               // has  already been declared

Let's see const in action.

When we declare variable as const we have to initialise it's value if not initialised it throws error.


copy
const c;
console.log(c)  // Uncaught SyntaxError: Missing
               //  initializer in const declaration

Can the const variable be redeclared as in it is in var.


copy
const c = 1;
console.log(c)  // 1
const c = 2;
console.log(c)  // Uncaught SyntaxError: Identifier 'c'
               //  has already been declared

const variable cannot be redeclared as in var

Now let's try to reinitialise const variable.


copy
const c = 1
console.log(c)  // 1
c = 2
console.log(c) // Uncaught TypeError: Assignment to
              //  constant variable.

const variable cannot be reinitialised.

That's all for today.