• JavaScript Video Tutorials

JavaScript - Void Keyword

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void is an important keyword in JavaScript which can be used as a unary operator that appears before its single operand, which may be of any type. This operator specifies an expression to be evaluated without returning a value.

Syntax

The syntax of void can be either of the following two −

<head>
   <script type = "text/javascript">
      <!--
         void func()
         javascript:void func()
         or:
         void(func())
         javascript:void(func())
      //-->
   </script>
</head>

Example 1

The most common use of this operator is in a client-side javascript: URL, where it allows you to evaluate an expression for its side-effects without the browser displaying the value of the evaluated expression.

Here the expression alert ('Warning!!!') is evaluated but it is not loaded back into the current document −

Live Demo 
<html>
   <head>      
      <script type = "text/javascript">
         <!--
         //-->
      </script>   
   </head>
   
   <body>   
      <p>Click the following, This won't react at all...</p>
      <a href = "javascript:void(alert('Warning!!!'))">Click me!</a>     
   </body>
</html>

Output

Example 2

Take a look at the following example. The following link does nothing because the expression "0" has no effect in JavaScript. Here the expression "0" is evaluated, but it is not loaded back into the current document.

Live Demo 
<html>
   <head>   
      <script type = "text/javascript">
         <!--
         //-->
      </script>      
   </head>
   
   <body>   
      <p>Click the following, This won't react at all...</p>
      <a href = "javascript:void(0)">Click me!</a>      
   </body>
</html>

Output

Example 3

Another use of void is to purposely generate the undefined value as follows.

Live Demo 
<html>
   <head>      
      <script type = "text/javascript">
         <!--
            function getValue() {
               var a,b,c;
               
               a = void ( b = 5, c = 7 );
               document.write('a = ' + a + ' b = ' + b +' c = ' + c );
            }
         //-->
      </script>      
   </head>
   
   <body>
      <p>Click the following to see the result:</p>
      <form>
         <input type = "button" value = "Click Me" onclick = "getValue();" />
      </form>     
   </body>
</html>

Output

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