Defiing Variables and Constants

Javascript Course

• Variable Types
• Defining and Using Constants

In a script (or program) we use constants and variables datas. The variables can change their values during program execution. These data are called 'variables'.
- A variable is a name of a location in computer memory, used to store data.

Using variables

In a script (or program) we use variables and constants datas. The variables can change their values The simplest way to use and refer to a variable is to write it. The name of the variable permits the access at its value and also can change the value if necessary.

var name ='Value';
//Sau
let name ='Value';

<h4>Example JS variable</h4>

<script>
let str ='Some text';
document.write(str);
</script>

The difference between var and let

The difference between var and let is scoping.

• A variable defined using a var statement is known throughout whole scope or the function it is defined in, from the start of the function.
  
• With var you can define a variable globally, that can be called and modified in functions.
• A variable defined using a let statement is only known in the block it is defined in, from the moment it is defined onward.
• The let variable is not added to the global window object.

<script>
let xn = 1;

if(xn ==1){
 let xn =2;
}

document.write(xn); // 1
</script>

<script>
var xn = 1;

if(xn ==1){
 var xn =2;
}

document.write(xn); // 2
</script>

Variable Types

There are several types of data that can be assigned to variables. The type of the value it determines the type of the variable.

• string - A text string, a series of characters added between single or double quotes.
  var x = 'Web Site CoursesWeb.net';
• number - Numbers can be written with, or without decimals.

  var x = 8;
  var y = 7.95;

• booleen - Can be one of these values: true or false.

  var x = true;
  var y = false;

• array - Several values separated by commas, ordered with a numeric index, starting from 0.

  let x = ['ab', 78, 'xy'];
  alert(x[1]); // 78

• object - A data object that may contain properties and functions.
  var x = {prop1:'ab', p2:78};
• function - A function that can return a value (with return).

  var fun = function(){
   return 'Value';
  };

• undefined - A variable without a value.
  let x;
• null - Null value, nulled.
  var x = null;

<script>
var x; //now is undefined

x = 5; //now is a number
document.write(x);

x = '<h4>MarPlo</h4>'; //now is a string
document.write(x);
</script>

Notice that the 'string' values (consisting of letters) are written between quotation marks (simples or doubles), and the 'number' can be written without quotation marks.

<script>
//a function
function f(){
 var x ='val';
}

document.write(x); //Error: x is not defined
</script>

Defining and Using Constants

Constants are defined with the const declaration.
Unlike variables, the value of a constant can not be changed and can not be redefined, its value remains the same, fixed.

const X = 'MarPlo';

//trying to change the value it results error in browser console
X ='abc';

//redefining, it results error in browser console
const X = 123;

<script>
const TJC = 'JavaScript Tutorial - Constants';
document.write(TJC);
</script>