Contrarily to what the currently accepted answer suggests, there’s nothing specific to PhantomJS when it comes to the difference between having WebDriver do a click and doing it in JavaScript.
The Difference
The essential difference between the two methods is common to all browsers and can be explained pretty simply:
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WebDriver: When WebDriver does the click, it attempts as best as it can to simulate what happens when a real user uses the browser. Suppose you have an element A which is a button that says “Click me” and an element B which is a
divelement which is transparent but has its dimensions andzIndexset so that it completely covers A. Then you tell WebDriver to click A. WebDriver will simulate the click so that B receives the click first. Why? Because B covers A, and if a user were to try to click on A, then B would get the event first. Whether or not A would eventually get the click event depends on how B handles the event. At any rate, the behavior with WebDriver in this case is the same as when a real user tries to click on A. -
JavaScript: Now, suppose you use JavaScript to do
A.click(). This method of clicking does not reproduce what really happens when the user tries to click A. JavaScript sends theclickevent directly to A, and B will not get any event.
Why a JavaScript Click Works When a WebDriver Click Does Not?
As I mentioned above WebDriver will try to simulate as best it can what happens when a real user is using a browser. The fact of the matter is that the DOM can contain elements that a user cannot interact with, and WebDriver won’t allow you to click on these element. Besides the overlapping case I mentioned, this also entails that invisible elements cannot be clicked. A common case I see in Stack Overflow questions is someone who is trying to interact with a GUI element that already exists in the DOM but becomes visible only when some other element has been manipulated. This sometimes happens with dropdown menus: you have to first click on the button the brings up the dropdown before a menu item can be selected. If someone tries to click the menu item before the menu is visible, WebDriver will balk and say that the element cannot be manipulated. If the person then tries to do it with JavaScript, it will work because the event is delivered directly to the element, irrespective of visibility.
When Should You Use JavaScript for Clicking?
If you are using Selenium for testing an application, my answer to this question is “almost never”. By and large, your Selenium test should reproduce what a user would do with the browser. Taking the example of the drop down menu: a test should click on the button that brings up the drop down first, and then click on the menu item. If there is a problem with the GUI because the button is invisible, or the button fails to show the menu items, or something similar, then your test will fail and you’ll have detected the bug. If you use JavaScript to click around, you won’t be able to detect these bugs through automated testing.
I say “almost never” because there may be exceptions where it makes sense to use JavaScript. They should be very rare, though.
If you are using Selenium for scraping sites, then it is not as critical to attempt to reproduce user behavior. So using JavaScript to bypass the GUI is less of an issue.