Friday, April 29, 2016

Control Statements in JavaScript

A conditional statement is a set of instructions that executes depending upon the condition is true or false.

As per MDN, JavaScript supports two conditional statements if…else and switch case.

JavaScript is a computer language mainly used for making Interactive websites under Web development. 
if statement:
Statements inside if will be executed if the expression is true. If the expression is false then no statements will be executed. We will be using comparison operators to evaluate the expression.
 
Syntax:
 
if (expression)
{
          statements will be executed only if the expression is true
}
Example:
<html>
<body>
      <script type="text/javascript">
         var age = prompt('Enter your age plz');
         if( age >=18 ){
         alert("Qualifies for driving");
         }
      </script>
</body>
</html>

Result:
Enter your age plz: 25
Qualifies for driving.
If we enter age greater than 18 then it will execute the statement otherwise will come out.

if ….else statement
It is the next form of control statements which allows to execute statements depending upon the expression is true or false.

Syntax:

if (expression)
{
           statements will be executed only if the expression is true
}
else {
          statements will be executed only if the expression is false
}
 
Example:
<html>
<body>
      <script type="text/javascript">
      var age = prompt('Enter ur age plz');
      if( age >=18 ){
      alert("Qualifies for driving");
      }
      else{
        alert(“Doesn’t qualify for driving”);
      }
       </script>
</body>
</html>
 
Result:
Enter your age plz: 25
Qualifies for driving
Enter your age plz: 15
Doesn’t qualify for driving

If age is greater than or equal to 18 then it will execute the if statement otherwise will execute the else statement.

if else if statement:

This is more advanced form of if..else that will execute statements on the basis of given conditions. It is useful in the case of multiple conditions. In this case only the first logical condition which is true will be executed.This can also be known as nested if statements.

Syntax:
if (expression1){
        statements will be executed only if the expression2 is true
}
else if (expression2){
       statements will be executed only if the expression2 is true
}
else if (expression3){
       statements will be executed only if the expression3 is true
}
else {
      statements will be executed only if the expression is false
}

Example:
<html>
<body>
      <script type="text/javascript">
      var book = prompt('Enter bookname');
      if( book = ‘Kiterunner’ ){
      alert("It comes under fiction category");
      }
      else if( book = ‘Wasted in Engineering’ ){
      alert("It comes under non fiction category");
      }
      else if( book = ‘A history of India’ ){
      alert("It comes under history category");
      }
      else{
     alert(‘Unknown category’);
      }
      </script>
</body>
</html>
 
Result:

It will display the result on the basis of the choices given. If we will enter Kiterunner in the prompt box, thenIt comes under fiction category’ will be the output. If we will enter any bookname which is not mentioned, example ‘Madhushala’, then result will be ‘unknown category’. For comparison use == (equal operator). For comparison don’t use =. ‘=’ is used for assigning values not for comparing them.

SWITCH CASE
The main objective of switch case is to execute different statements given on the basis of expression given. Each case statement will be checked against the expression given until a match is found. If nothing founds default statement will be executed. Break is very important after each statement. If we will not give it then every statement will be executed.
Syntax:
switch (expression)
{
    case condition 1: statement(s)
    break;
    case condition 2: statement(s)
    break;
    case condition n: statement(s)
    break;
    default: statement(s)
}
 
Example:

<!DOCTYPE html>
<html>
<head>
           <title>Switch Case</title>
</head>
<body>
<script type="text/javascript">
      var time = new Date();
      var myday = time.getDay();
      //alert(myday);
      switch(myday){
         case 0:
         alert('it is sunday');
         break;
         case 1:
         alert('it is monday')
         break;
         case 2:
         alert('it is tuesday')
         break;
         case 3:
         alert('it is wednesday')
         break;
        case 4:
        alert('it is thursday')
        break;
        case 5:
        alert('it is friday')
        break;
        case 6:
        alert('it is saturday')
        break;
       default:break;
       }
</script>
</body>
</html>

In the above example, new Date() function will display the current date and date with time zone. 
 
In the above example, new Date() function will display the current date and date with time zone. The getDay() function in JavaScript returns the weekday as a number between 0 and 6. (Sunday=0, Monday=1, Tuesday=2, Wednesday =3, Thursday=4, Friday=5, Saturday=6). Result will be current day and it will execute the case depending upon on which day it has been executed. For example. If we are executing it today, i.e. 21st April 2016. It will find the match with Case 4 and execute it. Result will be “it is Thursday”.

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