Window.alert() method displays an alert dialog with the optional specified content and an OK button.
messageis an optional string of text you want to display in the alert dialog, or, alternatively, an object that is converted into a string and displayed.
The alert dialog should be used for messages which do not require any response on the part of the user, other than the acknowledgement of the message.
The following text is shared between this article, DOM:window.prompt and DOM:window.confirm Dialog boxes are modal windows - they prevent the user from accessing the rest of the program's interface until the dialog box is closed. For this reason, you should not overuse any function that creates a dialog box (or modal window).
Starting with Chrome 46.0 this method is blocked inside an
<iframe> unless its sandbox attribute has the value
The argument is now optional as required by the spec.
|HTML Living Standard
The definition of 'alert()' in that specification.