[javascript] What does "javascript:void(0)" mean?

<a href="javascript:void(0)" id="loginlink">login</a>

I've seen such hrefs many times, but I don't know what exactly that means.

In addition to the technical answer, javascript:void means the author is Doing It Wrong.

There is no good reason to use a javascript: pseudo-URL(*). In practice it will cause confusion or errors should anyone try things like ‘bookmark link’, ‘open link in a new tab’, and so on. This happens quite a lot now people have got used to middle-click-for-new-tab: it looks like a link, you want to read it in a new tab, but it turns out to be not a real link at all, and gives unwanted results like a blank page or a JS error when middle-clicked.

<a href="#"> is a common alternative which might arguably be less bad. However you must remember to return false from your onclick event handler to prevent the link being followed and scrolling up to the top of the page.

In some cases there may be an actual useful place to point the link to. For example if you have a control you can click on that opens up a previously-hidden <div id="foo">, it makes some sense to use <a href="#foo"> to link to it. Or if there is a non-JavaScript way of doing the same thing (for example, ‘thispage.php?show=foo’ that sets foo visible to begin with), you can link to that.

Otherwise, if a link points only to some script, it is not really a link and should not be marked up as such. The usual approach would be to add the onclick to a <span>, <div>, or an <a> without an href and style it in some way to make it clear you can click on it. This is what StackOverflow [did at the time of writing; now it uses href="#"].

The disadvantage of this is that you lose keyboard control, since you can't tab onto a span/div/bare-a or activate it with space. Whether this is actually a disadvantage depends on what sort of action the element is intended to take. You can, with some effort, attempt to mimic the keyboard interactability by adding a tabIndex to the element, and listening for a Space keypress. But it's never going to 100% reproduce the real browser behaviour, not least because different browsers can respond to the keyboard differently (not to mention non-visual browsers).

If you really want an element that isn't a link but which can be activated as normal by mouse or keyboard, what you want is a <button type="button"> (or <input type="button"> is just as good, for simple textual contents). You can always use CSS to restyle it so it looks more like a link than a button, if you want. But since it behaves like a button, that's how really you should mark it up.

(*: in site authoring, anyway. Obviously they are useful for bookmarklets. javascript: pseudo-URLs are a conceptual bizarreness: a locator that doesn't point to a location, but instead calls active code inside the current location. They have caused massive security problems for both browsers and webapps, and should never have been invented by Netscape.)

Usage of javascript:void(0) means that the author of the HTML is misusing the anchor element in place of the button element.

Anchor tags are often abused with the onclick event to create pseudo-buttons by setting href to "#" or "javascript:void(0)" to prevent the page from refreshing. These values cause unexpected behavior when copying/dragging links, opening links in a new tabs/windows, bookmarking, and when JavaScript is still downloading, errors out, or is disabled. This also conveys incorrect semantics to assistive technologies (e.g., screen readers). In these cases, it is recommended to use a <button> instead. In general you should only use an anchor for navigation using a proper URL.

Source: MDN's <a> Page.

The void operator evaluates the given expression and then returns undefined. It avoids refreshing the page.

There is a huge difference in the behaviour of # vs javascript:void(0);.

# scrolls you to the top of the page but javascript:void(0); does not.

This is very important if you are coding dynamic pages because the user does not want to go back to the top when they click a link on the page.

To understand this concept one should first understand the void operator in JavaScript.

The syntax for the void operator is: void «expr» which evaluates expr and returns undefined.

If you implement void as a function, it looks as follows:

function myVoid(expr) {
    return undefined;
}

This void operator has one important usage that is - discarding the result of an expression.

In some situations, it is important to return undefined as opposed to the result of an expression. Then void can be used to discard that result. One such situation involves javascript: URLs, which should be avoided for links, but are useful for bookmarklets. When you visit one of those URLs, many browsers replace the current document with the result of evaluating the URLs “content”, but only if the result isn’t undefined. Hence, if you want to open a new window without changing the currently displayed content, you can do the following:

javascript:void window.open("http://example.com/")

It means it’ll do nothing. It’s an attempt to have the link not ‘navigate’ anywhere. But it’s not the right way.

You should actually just return false in the onclick event, like so:

<a href="#" onclick="return false;">hello</a>

Typically it’s used if the link is doing some ‘JavaScript-y’ thing. Like posting an AJAX form, or swapping an image, or whatever. In that case you just make whatever function is being called return false.

To make your website completely awesome, however, generally you’ll include a link that does the same action, if the person browsing it chooses not to run JavaScript.

<a href="backup_page_displaying_image.aspx"
   onclick="return coolImageDisplayFunction();">hello</a>

Web Developers use javascript:void(0) because it is the easiest way to prevent the default behavior of a tag. void(*anything*) returns undefined and it is a falsy value. and returning a falsy value is like return false in onclick event of a tag that prevents its default behavior.

So I think javascript:void(0) is the simplest way to prevent the default behavior of a tag.

It's worth mentioning that you'll sometimes see void 0 when checking for undefined, simply because it requires fewer characters.

For example:

if (something === undefined) {
    doSomething();
}

Compared to:

if (something === void 0) {
    doSomething();
}

Some minification methods replace undefined with void 0 for this reason.

You should always have an href on your a tags. Calling a JavaScript function that returns 'undefined' will do just fine. So will linking to '#'.

Anchor tags in Internet Explorer 6 without an href do not get the a:hover style applied.

Yes, it is terrible and a minor crime against humanity, but then again so is Internet Explorer 6 in general.

I hope this helps.

Internet Explorer 6 is actually a major crime against humanity.

From what I've seen, the void operator has 3 common uses in JavaScript. The one that you're referring to, <a href="javascript:void(0)"> is a common trick to make an <a> tag a no-op. Some browsers treat <a> tags differently based on whether they have a href , so this is a way to create a link with a href that does nothing.

The void operator is a unary operator that takes an argument and returns undefined. So var x = void 42; means x === undefined. This is useful because, outside of strict mode, undefined is actually a valid variable name. So some JavaScript developers use void 0 instead of undefined. In theory, you could also do <a href="javascript:undefined"> and it would so the same thing as void(0).

void is an operator that is used to return a undefined value so the browser will not be able to load a new page.

Web browsers will try and take whatever is used as a URL and load it unless it is a JavaScript function that returns null. For example, if we click a link like this:

<a href="javascript: alert('Hello World')">Click Me</a>

then an alert message will show up without loading a new page, and that is because alert is a function that returns a null value. This means that when the browser attempts to load a new page it sees null and has nothing to load.

An important thing to note about the void operator is that it requires a value and cannot be used by itself. We should use it like this:

<a href="javascript: void(0)">I am a useless link</a>

It is a very popular method of adding JavaScript functions to HTML links.
For example: the [Print] links that you see on many webpages are written like this:

<a href="javascript:void(0)" onclick="callPrintFunction()">Print</a>

Why do we need href while onclick alone can get the job done? Because when users hover over the text 'Print' when there's no href, the cursor will change to a caret (ꕯ) instead of a pointer (👆). Only having href on an a tag validates it as a hyperlink.

An alternative to href="javascript:void(0);", is the use of href="#". This alternative doesn't require JavaScript to be turned on in the user's browser, so it is more compatible.

A link must have an href target to be specified to enable it to be a usable display object.

Most browsers will not parse advanced JavaScript in the href of an <a> element, for example:

<a href="javascript:var el = document.getElementById('foo');">Get element</a>

Because the href tag in most browsers does not allow whitespace or will convert whitespace to %20 (the HEX code for space), the JavaScript interpreter will run into multiple errors.

So if you want to use an <a> element's href to execute inline JavaScript, you must specify a valid value for href first that isn't too complex (doesn't contain whitespace), and then provide the JavaScript in an event attribute tag like onClick, onMouseOver, onMouseOut, etc.

The typical answer is to do something like this:

<a href="#" onclick="var el = document.getElementById('foo');">Get element</a>

This works fine but it makes the page scroll to the top because the # in the href tells the browser to do this.

Placing a # in the <a> element's href specifies the root anchor, which is by default the top of the page, but you can specify a different location by specifying the name attribute inside an <a> element.

<a name="middleOfPage"></a>

You can then change your <a> element's href to jump to middleOfPage and execute the JavaScript in the onClick event:

<a href="#middleOfPage" onclick="var el = document.getElementById('foo');">Get element</a>

There will be many times where you do not want that link jumping around, so you can do two things:

<a href="#thisLinkName" name="thisLinkCame" onclick="var elem = document.getElementById('foo');">Get element</a>

Now it will go nowhere when clicked, but it could cause the page to re-centre itself from its current viewport.

The best way to use in-line javascript using an <a> element's href, but without having to do any of the above is JavaScript:void(0);:

<a href="javascript:void(0);" onclick="var el = document.getElementById('foo');">Get element</a>

This tells the browser no to go anywhere, but instead execute the JavaScript:void(0); function in the href because it contains no whitespace, and will not be parsed as a URL. It will instead be run by the compiler.

void is a keyword which, when supplied with a parameter of 0 returns undefined, which does not use any more resources to handle a return value that would occur without specifying the 0 (it is more memory-management/performance friendly).

The next thing that happens is the onClick gets executed. The page does not move, nothing happens display-wise.