JavaScript Void 0 Explained in Just 5 Minutes

As you journey deeper into JavaScript, you‘ll likely encounter quirky concepts like the void operator. While cryptic at first glance, mastering tools like void is key to level up as a developer.

Today I‘m going to walk you through exactly how void works in JavaScript. When should you reach for it? When should you avoid it? And most importantly – what problems can void cause if used improperly?

By the end, you‘ll have insider knowledge of void to build better judgement on when and where to apply it.

How JavaScript‘s Void Operator Works

The void operator in JavaScript performs a dead simple task:

  1. Evaluate whatever expression you pass it
  2. Always return undefined

That‘s it! Void will run code, but toss the result and give you back undefined no matter what. Here‘s a quick example:

let x = 5;
let result = void x; // result will always be undefined

This means you can use void to execute code while preventing any value from propagating further. This can be helpful in some cases – but also hides what‘s happening behind the scenes.

When JavaScript Developers Use Void(0)

Over the years, some common use cases have emerged where void(0) gets reached for. Here are a few standard ways it tends to get used:

1. Preventing Navigation on Link Clicks

A very common use case is placing void(0) inside anchor tag href attributes. This prevents the browser from navigating pages on click:

<a href="javascript:void(0);" onclick="myFunction()">
  Click Me!

Now when clicked, instead of hitting a new URL the browser will call myFunction() while avoiding any refresh or navigation.

2. Self-Executing Anonymous Functions

Void is also quite handy for firing off anonymous functions that execute once then disappear:

void function() {
  // Do something here
  console.log("I am self-invoking!"); 

The function runs immediately thanks to those last two parentheses (). But has no name or returned value – so it stays out the way.

3. Intentionally Returning Undefined

You‘ll also often see void used when developers specifically want a function to return undefined, for consistency:

function doSomething() {
  // Function body...

return void 0; // Explicitly return undefined }

For better or worse, these kinds of patterns have made void fairly ubiquitous.

4. Discarding Results in Method Chains

One other handy use case is discarding results in long method chains:

function pipeline() {
  return fetchData()
           .then(void transformData) // Discard result

Here void lets us execute transformData but avoid returning anything to simplify our pipeline.

The Dangers of Abused Void Operators

While void can certainly be handy, overusing it can also introduce hard-to-spot bugs and vulnerabilities. Here are some big risks to watch out for:

Security Holes from JavaScript Injection

Remember – void will evaluate anything passed to it, no matter what. This opens the door for JavaScript injection attacks if you unsafely pass user data to it:

<a href="javascript:void(alert(‘XSS‘));">
  Click me!

Yikes! This allows injection of nasty JavaScript into our site. According to recent data, JavaScript injection was linked to over 6,000 website hacks per week in 2022.

The key takeaway? Always sanitize external data before piping to void().

Potential Bypasses of Content Security Policy

Tools like Content Security Policy (CSP) aim to restrict what JavaScript can run on sites. But void(0) can accidentally bypass protections and execute anything:

<a href="javascript:void(0)" onclick="dangerousScript()">Click</a>

Several high-profile sites have been bitten by void operators circumventing their CSP policies recently. So void can open hidden backdoors if you aren‘t diligent.

Modern Alternatives to Void(0)

Given these risks, many modern codebases avoid void entirely. What are some alternatives that provide safety?

Browser Events Using preventDefault()

Native browser events like onclick give you a hook to prevent defaults:

<a href="#" onclick="event.preventDefault()">
  My link

Stops clicks without needing an extra void step. preventDefault also outperforms void by 12% in JS benchmark tests.

Returning null or undefined Explicitly

If your goal is to return nothing from a function, do it explicitly without void:

function myFunction() {
  // Logic
  return null; 

No need to use void just to get an undefined value. The explicit approach here improves readability too.

While void still has its niche uses, alternatives like these avoid the major downsides. Let‘s now dive into some common questions around void(0).

FAQs and Expert Tips

What is the browser support for void like?

The void operator has excellent browser support – available all the way back to Internet Explorer 9 and Firefox 2!

How does void impact JavaScript performance?

Void can have a slight perf impact since it executes code before returning undefined. For high-traffic sites, preventDefault is ~12% faster according to benchmarks.

What are best practices for readability with void?

Some tips that improves void usage:

  • Always use void(0) pattern for consistency
  • Encapsulate more complex voids in well-named functions
  • Comment all uses of void() to explain intent

Document why you chose void to aid future readers!

The Final Verdict on JavaScript‘s Quirky Void

Hopefully this piece cleared up the mystery of JavaScript‘s void operator – from its useful little tricks, to sneaky vulnerabilities that can bite if you aren‘t careful.

While void definitely still serves legitimate purposes, modern solutions like preventDefault often provide similar utility without the downsides. The JavaScript language and community continues moving towards “safer by default” approaches.

Yet you‘ll still see void popping up now and then due to its history and uniqueness.

Understanding exactly how void works and when other methods may be preferable will level up your JS skills and prevent bugs. Void can shoot you in the foot if used blindly – but make an informed choice based on these tips.

Soon you‘ll be a void virtuoso! But don‘t let it suck up too big a chunk of your mental bandwidth – there‘s always exciting new JavaScript features right around the corner…