Variable in Javascript

How to declare variable in javascript?

variable is used to store data.

Synatx

keyword valriableName;

Example

let age;

In The above example we can use let keyword and declare one variable name of that variable is age. This is declaration part of in the variable.

age = 23;

In The above example we assign 23 value for age variable. This 23 value are store in age variable. This is assignment part of in the variable.

Example

let age = 23;

In the above example we create declaration and assignment part at same time.

There are four Ways to declare variable in javascript:

Keyword

var
let
const

let

Let keyword used to declare variables in JavaScript. Variables defined using let cannot be re-declared.

Code

<!DOCTYPE html>
<html>
  <head>
    <script>
      let age = 20;
      document.write(age);
    </script>
  </head>
  <body></body>
</html>

Output

20

In the above example we can create one variable using let keyword name of that variable age. we store 20 value in age variable. In next line we print value of document.write(age); age variable.

Code

<!DOCTYPE html>
<html>
  <head>
    <script>
      let age = 20;
      age = 50;
      document.write(age);
    </script>
  </head>
  <body></body>
</html>

Output

50

In the above example we can create one variable using let keyword name of that variable age. we store 20 value in age variable. In next line we store 50 value in same variable. In next line we print value of document.write(age); age variable. But the output of above eaxmple is 50 because one variable store only one value at a time.

Code

<!DOCTYPE html>
<html>
  <head>
    <script>
      let age = 20;
      document.write("Age: " + age);
    </script>
  </head>
  <body></body>
</html>

Output

Age: 20

Example

<!DOCTYPE html>
<html>
  <head>
    <script>
      let age;
      document.write("Age: " + age);
    </script>
  </head>
  <body></body>
</html>

Output

Age: undefined

const

Example

<!DOCTYPE html>
<html>
  <head>
    <script>
      const age = 22;
      document.write("Age: " + age);
    </script>
  </head>
  <body></body>
</html>

Output

Age: 22

Example

<!DOCTYPE html>
<html>
  <head>
    <script>
      let val1 = 10;
      let val2 = 20;
      let sum = val1 + val2;
      document.write(sum);
    </script>
  </head>
  <body></body>
</html>

Output

30

In the above example we can create one variable using let keyword name of that variable val1 and we store 10 value in that variable. Next line we create second variable using let keyword name of that variable val2 and we store 20 value in that variable. Third line we create sum variable and in that variable we store addition of val1 + val2 variable. In next line we print value of document.write(sum); sum variable.

Difference between var, let and const

var

The scope of the var variable is the functional scope.
It can be updated and re-declared in scope.
It can be declared without initialization.
It can be accessed without initialization because its default value is undefined.

let

Let allow us to declare a variable that is limited to the scope of a block.
In let first we declare variable and second time we used so there is no need to write second time.
It can be declared without initialization.
It cannot accessed without initialization, or a "reference error" will be raised.

const

The scope of the const variable is block scope.
It cannot be updated or re-declared in scope.
It cannot be accessed without initialization because it cannot be declared without initialization.

How to declare variable in javascript?​

let​

const​

Difference between var, let and const​

var​

let​

const​

How to declare variable in javascript?

let

const

Difference between var, let and const

var

let

const