JavaScript execution in WebDriver, JavascriptExecutor Interface, executeScript

public interface JavascriptExecutor :

The interface indicates the driver can execute JavaScript.
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////The below example type selenium in google and click search

import org.openqa.selenium.JavascriptExecutor;
import org.openqa.selenium.firefox.FirefoxDriver;

public class google_js
{
  public static void main(String[] args) throws Exception
  {
   FirefoxDriver fd=new FirefoxDriver();
   fd.get("http://www.google.com");
   Thread.sleep(5000);
         ///create object for JavascriptExecutor interface
   JavascriptExecutor js=(JavascriptExecutor)fd;
         ////get the element in the page and perform the required event
   js.executeScript("window.document.getElementById('gbqfq').value='selenium' ");
   js.executeScript("window.document.getElementById('gbqfb').click() ");
  }
}
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executeScript method :
Executes JavaScript in the the current selected frame or window.

If the script has a return value (i.e. if the script contains a return statement), then the following steps will be taken:

    For an HTML element, this method returns a WebElement
    For a decimal, a Double is returned
    For a non-decimal number, a Long is returned
    For a boolean, a Boolean is returned
    For all other cases, a String is returned.
    For an array, return a List