Tweak .indexOf() to make 'Grocery List' program more user friendly?

Dave had us construct a program using Arrays, Conditional Statements and Array Methods to have a prompt come up that allows users to check the stock of a grocery store. Code looks like this:

var inStock = [ 'apples', 'eggs', 'milk', 'cookies', 'cheese', 'bread', 'lettuce', 'carrot', 'broccoli', 'pizza', 'potato', 'crackers', 'onion', 'tofu', 'frozen dinner', 'cucumber'];
var search;

function print(message) {
  document.write( '<p>' + message + '</p>');
}

while ( true ) {
  search = prompt("Search for a product in our store. Type 'list' to show all of the produce and 'quit' to exit");
  search = search.toLowerCase();
  if (search === 'quit' ) {
    break;
  } else if (search === 'list' ) {
    print(inStock.join(', '));    
  } else {
    if ( inStock.indexOf( search ) > -1 ) {
      print( 'Yes, we have ' + search + ' in stock.');
  } else {
      print ("No, we don't have " + search + " in stock."); 
  }
 }                  
}

At the end of this video he inserts the search = search.toLowerCase(); statement to make the program more user friendly: in case the user typed their responses in all or mixed caps, the program would still list those items as 'in stock' as long as the spelling is the same as something in the array.

For fun I'd like to further this - so that in case a user types a plural form of an item which is listed as singular in the array - or vice versa - the program would still give the 'correct' response. So if the user typed carrots, the program will still respond that the item (listed as 'carrot' in the array) is in stock. Besides adding 'carrots' (plural) to the array, is there better way to do this?