If and else statements in Javascript

Using if and else statements

The ordinary if/else statement

  • Programs needs to take action based on a true condition, or another action based on a false condition.
  • For this we are using if and else statements which also both can be nested:
    With if and else With if, else if and else
      if (expression1) {
        if (expression2)
          statement1;
        else {
          if (expression3)
            statement2;
          else
            statement3;
        }
      }
      else
        statement4;
    if (expression1) {
        if (expression2)
          statement1;
        else if (expression3)
            statement2;
         else
            statement3;
      }
      else
        statement4;
    
    
    
    • If the expression followed by the if statements evaluates to true, statements after the if statement will be executed else the statements after a else starts executing.
    • If more than one statement exists for either the true or the false part you need to surround the statements with braces ({}).
    if & else example:
    <script type="text/javascript">
      // Demonstrates if/else and
      // proper use of braces
      // in nested if statements
      var number=4;
      number=prompt("Enter a number less than \n"
        +"5 or greater than 20:");
      if (number >= 5) {
        if (number > 20)
          document.write("Number is greater than 20!<br>");
      }
      else
        document.write("Number is less than 5!<br>");
    </script>

The Conditional (Ternary) Operator.

  • This is a short notation to use for a simple if & else statement:
    (expression1) ? (expression2) : (expression3)

    This is: if expression1 is true, return the value of expression2; otherwise, return the value of expression3.
    <script type="text/javascript">
      var num1=5, num2=4 ;
      var z = (num1 > num2) ? "num1>num2" : "num1<=num2";
      document.write(z);
    </script>
    

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