JavaScript | Comments

Last Updated : 01 Mar, 2019

Comments are used to prevent the execution of statements. Comments are ignored while the compiler executes the code. Comments are user friendly as user can get explanations of code using comments.

Syntax:

// For single line comment

/* For block of lines comment
...
...
*/

Return Value: During execution of code, comments are ignored.

Example 1: This example illustrates the single line comment using //.

<!DOCTYPE html>
<html>
  
<head>
    <title>
        JavaScript Comments
    </title>
      
    <script>
          
        // Function to add two numbers
        function add() {
              
            // Declare three variables
            var x, y, z;
              
            // Input a number and store it into variable x
            x = Number( document.getElementById("num1").value );
              
            // Input a number and store it into variable x
            y = Number( document.getElementById("num2").value );
              
            // Sum of two numbers
            z= x +y;
              
            // Return the sum
            document.getElementById("sum").value = z;
        }
    </script>
</head>
  
<body>
    Enter the First number: <input id="num1"><br><br>
    Enter the Second number: <input id="num2"><br><br>
      
    <button onclick="add()">
        Sum
    </button>
      
    <input id="sum">
</body>
  
</html>                    

Output:
Before clicking on the button:

After clicking on the button:

Example 2: This example illustrates the multi-line comment using /* … */

<!DOCTYPE html>
<html>
  
<head>
    <title>
        JavaScript Comments
    </title>
      
    <script>
      
        /* Script to get two input from user
        and add them */
        function add() {
              
            /* Declare three variable */
            var x, y, z;
              
            /* Input the two nos. num1, num2
            Input num1 and store it in x
            Input num2 and store it in y
            The sum is stored in a variable z*/
            x = Number(document.getElementById("num1").value); 
            y = Number(document.getElementById("num2").value);
            z = x + y;
              
            document.getElementById("sum").value = z;
        }
    </script>
</head>
  
<body>
    Enter the First number : <input id="num1"><br><br>
    Enter the Second number: <input id="num2"><br><br>
      
    <button onclick="add()">
        Sum
    </button>
      
    <input id="sum">
</body>
  
</html>                    

Output:
Before clicking on the button:

After clicking on the button:

Example 3: This example illustrates that commented code will never execute.

<!DOCTYPE html>
<html>
  
<head>
    <title>
        JavaScript Comments
    </title>
      
    <script>
          
        function add() {
              
            var x, y, z;
              
            x = Number( document.getElementById("num1").value );
              
            y = Number( document.getElementById("num2").value );
              
            z= x +y;
              
            // Comment the code section
            //document.getElementById("sum").value = z;
        }
    </script>
</head>
  
<body>
    Enter the First number: <input id="num1"><br><br>
    Enter the Second number: <input id="num2"><br><br>
      
    <button onclick="add()">
        Sum
    </button>
      
    <input id="sum">
</body>
  
</html>                    

Output:
Before clicking on the button:

After clicking on the button:

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