Error while executing javascript code in python3 while using selenium - python-3.x

Question

I was trying to remove the ad and other pop-up using selenium after opening the webpage. The pop-up is getting removed but the ad is not getting removed. There is some error in executing javascript code(using it remove ad). For which there is no reason given. Also, for the ad, when I open the webpage myself by typing the link then ad does not appear but when I run the program, in code generated browser ad appears(reason I don't know). I have attached both code and error. Here is the code
from selenium import webdriver
import time
from selenium.webdriver.common.by import By
driver = webdriver.Chrome()
driver.get("http://indianexpress.com/columnists/")
// To remove pop-up
later = driver.find_elements(By.CLASS_NAME, "iz_block_button")
later[0].click()
//to remove ad
all_iframes = driver.find_elements_by_tag_name("iframe")
if len(all_iframes) > 0:
print("Ad Found\n")
driver.execute_script("""
var elems = document.getElementsByTagName('iframe');
for(var i = 0, max = elems.length; i < max; i++)
{
elems[i].visibility=hidden;
}
""")
print('Total Ads: ' + str(len(all_iframes)))
else:
print('No frames found')
driver.close()
Error:-
Traceback (most recent call last):
File "/Users/arjungoyal/Desktop/untitled/a.py", line 23, in <module>
""")
File "/usr/local/lib/python3.6/site-packages/selenium/webdriver/remote/webdriver.py", line 627, in execute_script
'args': converted_args})['value']
File "/usr/local/lib/python3.6/site-packages/selenium/webdriver/remote/webdriver.py", line 312, in execute
self.error_handler.check_response(response)
File "/usr/local/lib/python3.6/site-packages/selenium/webdriver/remote/errorhandler.py", line 242, in check_response
raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.WebDriverException: Message: unknown error: hidden is not defined
(Session info: chrome=66.0.3359.181)
(Driver info: chromedriver=2.38.552518 (183d19265345f54ce39cbb94cf81ba5f15905011),platform=Mac OS X 10.13.4 x86_64)
Please someone tell what the error is and how to remove it.