How Pop-up Blockers Work

Before pop-up blockers, your entire screen could be overrun by ads like these. See more Web advertising images.

It's happened to almost all of us at some point in our Web surfing experience -- you're visiting a new site when all of a sudden your screen begins to fill with boxes advertising goods and services from every corner of the Internet. Marketing firms claim that pop-up ads get more clicks than static advertising banners, though that could be due to user error rather than user intent. In other words, someone trying to close a pop-up ad might accidentally click on the ad link, giving the advertiser a false positive.

Web Advertising Image Gallery


Some pop-up ads contain extensive animations or sound elements that can be distracting to surfers or even make it impossible to navigate through a Web page. Sometimes a user might prompt three more windows to open just as he manages to close the first one. Even worse, some pop-ups contain malicious code like spyware or computer viruses.

Most pop-up ads are Javascript applications that open a new browser window containing new content. The new window appears on top of your current view, obscuring the Web page you actually want to see. Because the ads are so intrusive, Internet users have consistently voted them the most annoying form of online advertisement in surveys.

Enter the pop-up blocker. The earliest pop-up blockers appeared on the Internet scene in the mid '90s. The then-most popular blocker was PopUp Killer, created by Xavier Flix. Today, pop-up blockers come packaged in most Web browsers, and dozens of commercial and free programs also keep your browsing window clutter-free.

In this article, we'll look at what a pop-up blocker does, the different kinds of blockers you can use to reduce pop-ups and the potential issues you might encounter when you've activated a pop-up blocker.



Pop-up Blocker Software

A typical pop-up ad

Most pop-up ads are programmed in the Javascript programming language. When you click on a Web page that has pop-up ads, the ads activate as the page loads into your browser. The code for the pop-ups can be found within the code for the page itself. Pop-up blockers are programmed to scan a Web page's source code and search for any signs of a program telling your browser to open a new window. If the blocker finds this code, it deactivates the command and you don't get any pop-ups.

Not all pop-ups open when the Web page loads, though. Some activate when you click on a link or when you scroll your mouse over an activation site on the Web page. A good pop-up blocker can detect the code for these ads as well, though some are only able to deactivate a pop-up after it starts to open. If you've ever seen a pop-up quickly appear and disappear when you have a pop-up blocker activated, that's what happened. The blocker detected the ad after it activated, then disabled it.


Some pop-ups use other programming languages, such as Flash. A Javascript pop-up blocker is helpless against flash pop-ups because it's a completely different programming language. To block Flash ads, you'll need Flashblock program. These programs are similar to pop-up blockers in that they disable the commands found in Web pages that activate Flash animation. You can find programs that incorporate Javascript and flash ad blockers in one package.

But not all pop-up blockers are created equal. Some of them are very good at blocking nuisance pop-ups while leaving helpful pop-ups alone -- they're Intelligent pop-up blockers. In general, most intelligent pop-up blockers only disable pop-ups that open when a Web page first loads into your browser -- any pop-up that opens as a result of the user clicking on a link remains unhampered.

Trainable pop-up blockers rely on user input to determine which pop-ups to deactivate and which to leave alone. No two pop-up blocking programs are exactly alike, but in general the blocker asks the user if he wants to disable a particular pop-up when it first appears on the user's screen. If the user clicks on "Yes," then the blocker puts the offending pop-up on a black list, or a list of ads that should be deactivated in the future.

If you choose "No," the blocker puts the pop-up on a white list, which means it won't attempt to block the pop-up window again. While this kind of blocker is very easy to customize, it can become frustrating for the user as he makes choices for each and every pop-up. Also, if the advertiser alters the original pop-up in some way, it may slip past the blocker.

Serial pop-up blockers block every pop-up ad they're capable of uncovering. These blockers seek out every variation of the code used to generate pop-up ads and deactivate it whether the pop-up contains advertising or not. While these blockers usually ensure the user a pop-up-free experience, they can also cause complications when navigating through certain Web pages.

In the next section, we'll look at why you would want to disable a pop-up blocker.


Pop-up Blocker Disadvantages

Pop-up ads are annoying at best and can considerably disrupt your Web surfing at worst. So why would we ever not want to block them? Some Web pages use the same code to open a new browser window that actually contains helpful information. Some online shopping sites use pop-ups to give users a close-up view of items.

Other sites might use pop-ups so that the user can fill out a form without leaving the main site. This is often the case when you try to download a file. Some pop-up blockers can't differentiate between these helpful pop-ups and the intrusive advertising we've all come to know and loathe.


If you use a trainable or intelligent pop-up blocker, you might be all right. Most of these blockers only prevent nuisance pop-ups, though no blocker is 100-percent effective. You're likely to see at least a few annoying ads no matter which blocker you use, and you're just as likely to miss out on some helpful pop-ups at some point.

Fortunately, just about every blocker program has a suite of options the user can tweak to ensure that his Web surfing isn't adversely affected. Some programs allow you to tell the blocker which Web sites are safe for pop-ups. Once you designate a Web site as safe, the blocker will no longer seek out and disable pop-up code on that site. Other blockers might require you disable the blocking procedure completely before you can view any pop-ups. It's important to remember to reactivate the blocker once you leave the site you're looking at -- otherwise you're back to being flooded with pop-ups.

To learn more about pop-up blockers and online advertising, check out the links on the next page.


Lots More Information

Related How Stuff Works Articles:

More Great Links

  • Adams, Cecil. "What's the deal with popup ads?" The Straight Dope.
  • Hamilton, Anita. "Stopping those pop-up ads." Time Magazine. September 10, 2001. Vol. 158, Issue 10. Page 87.
  • Ho, David. "Advertisers Ditch Pop-Ups for New Tricks." The Atlanta Journal-Constitution. December 4, 2005. Page C3.
  • "From Annoyance to Pleasure - The Artistic Popup Approach." May 12, 2006.
  • "Advertising Avoidance." New Scientist. December 2006. Vol. 192, Issue 2582. Page 23.
  • "Pop-up Blocker Shoot-Out." PC Today. April 2005, Vol. 3, Issue 4. Pages 85-88.
  • Pop-up Killer Review.