JavaScript: Explain about void operator

void operator evaluates an expression and return undefined.

Syntax

void expression

void (expression)

Since ‘void expression’ return undefined, you can use this while checking for undefined variables.

HelloWorld.js

var x;
var y = 10;

if(x === (void 0)){
  console.log("x is undefined");
}else{
  console.log("Value of x is : " +  x);
}

if(y === (void 0)){
  console.log("y is undefined");
}else{
  console.log("Value of y is : " + y);
}

Output

x is undefined

Value of y is : 10

Example 1: Create hyper link that does nothing when user clicks on it

<a href="javascript:void(0)">Click here to do nothing</a>

When the user clicks on hyper link, void(0) returns undefined, which has no effect in JavaScript.

Example 2: Submit the form when user clicks on hyper link

<a href="javascript:void(document.form.submit())">Submit the form</a>

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More About Methods

Methods without returning any value and without parameters

    Syntax:

    void methodName(){

        //body

    }

If a method don't return any value, then it's return type must be void

Example

class MethodEx{
 void print(){
  System.out.println("I am not returning any value");
 }
 
 public static void main(String args[]){ 
  MethodEx obj = new MethodEx();
  obj.print();  
 }
}  

Output

I am not returning any value

Methods without returning any value and with parameters

    Syntax:

    void methodName(parameters){

        //body

    }

    You can give any number of parameters to a method

 Example

class MethodEx{
 void print(int a, int b){ 
  System.out.println("The parameter value is " + a +" " + b);
 }
 public static void main(String args[]){
  MethodEx obj = new MethodEx();
  obj.print(10,20);
 } 
}

 

Output    

 The parameter value is 10 20

Methods with returning a value and without parameters

    Syntax

    returnType methodName(){

        //body

    }

Here returnType is any primitive or reference type. The method must returns the value which is compatible with the returnType of the method.

Example

class MethodEx{
 String fun(){ 
  return "Let's have fun with methods";
 }
 
 public static void main(String args[]){ 
  MethodEx obj = new MethodEx();
  String s = obj.fun();
  System.out.println(s);   
 }
}

Output

Let's have fun with methods

Observation

The method fun() return type is String, so whatever the value returns by the method fun(), we must store it in the String variable like “String s = obj.fun()”.

Methods with returning a value and with parameters

    Syntax

    returnType methodName(parameters){

        //body

    }

A method can have any number of parameters. Here returnType is any primitive or reference type. The method must returns the value which is compatible with the returnType of the method.

Example

class MethodEx{
 String fun(String firstName, String lastName, int age){
  return "I am " + firstName +" " + lastName + " and my Age is " + age;
 }
 
 public static void main(String args[]){
  MethodEx obj = new MethodEx();
  String s = obj.fun("Aravind", "Phaneendra", 25);
  System.out.println(s);
 }
}

        

Output

 I am Aravind Phaneendra and my Age is 25

    

Some Points to Remember

1. returning incompatible value causes the compiler to throw error.

     Example

class MethodEx{
 int sum(int a, int b){
  return 10.01;
 }
}

                  

When you try to compile the above program, compiler will throw the below error

MethodEx.java:4: error: possible loss of precision
 return 10.01;
 ^
 required: int
 found: double
1 error

2. You can call the method in print methods, if it is returning a value, i.e, non void type methods

    Example

class MethodEx{
 int sum(int a, int b){
  return (a+b);
 }
 public static void main(String args[]){
  MethodEx obj = new MethodEx();
  System.out.println("Sum of 10 and 20 is " + obj.sum(10,20));
 }
}

Output

Sum of 10 and 20 is 30

         

3. Calling void methods inside the print causes compile time error

     Example

class MethodEx{
 void print(){
 }
 
 public static void main(String args[]){
  MethodEx obj = new MethodEx();
  System.out.println(obj.print());
 }
} 

When you try to compile the above program, compiler will throw the below error.      

MethodEx.java:7: error: 'void' type not allowed here
 System.out.println(obj.print());
 ^
1 error

           

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