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JavaScript | Comments

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  • Difficulty Level : Medium
  • Last Updated : 26 Oct, 2021

Comments are used to prevent the execution of statements. Comments are ignored while the compiler executes the code. Comments are user-friendly as users 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 //.

html




<!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 /* … */

html




<!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.

html




<!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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