Editing Pop-up ad (section)

===Pop-under ads===
Pop-under ads are similar to pop-up ads, but the ad window appears hidden behind the main browser window rather than superimposed in front of it. As pop-up ads became more widespread and intrusive, often taking up the whole computer screen, many users would immediately close the pop-up ads that appeared over a site without looking at them. Pop-under ads do not immediately impede the view of content, but remain unnoticed until the user closes or minimizes the main browser window. 

====Pop-under ad technology====
A pop-under ad involves two JavaScript functions introduced in 1995 with the Netscape 2.0b3 browser. Modern web publishers and advertisers use it to create a window in front of the user's screen, load an advertisement, and then send it behind the screen.

<syntaxhighlight lang="javascript">
// create a new window in front of the current site
window.open(URL, windowName[, windowFeatures]); 
// push the loaded advertisement back behind the browser
window.focus();
</syntaxhighlight>

Most modern browsers allow <code>window.open</code> to execute only if it was called by a user interaction (e.g., a mouse click) [[event handler]]. Any non-interactive calls (timer callback, load events, etc.) to <code>window.open</code> result in the new window being blocked.

To bypass this restriction, most pop-under ads trigger on a mouse click event listener attached directly to the document or the document's body. This enables catching all mouse click events that were not consumed by other click event handlers, and calling <code>window.open</code> without being blocked. For example, when the user selects a text, the mouse click triggers the mouse click handler attached to the document and a pop-under opens using the above code. Other techniques to bypass the <code>window.open</code> call restriction do so by "hijacking" mouse clicks.