how to instantiate javascriptexecutor in selenium code example

Example 1: how to cast to javascript executor

JavascriptExecutor js = (JavascriptExecutor) driver;  
js.executeScript(Script,Arguments);

Example 2: how to use javascriptexecutor for loop in selenium c#

String script3 = "return document.getElementsByTagName('g')[" + f9 + "].textContent";
n = (String) ((JavascriptExecutor) driver).executeScript(script3, hiddenDiv);