What does the exclamation mark do before the function?

The sorcery behind ! in function expressions

When using function expressions, ! ensures that the function is instantly invoked minus any syntax errors. It's a stylistic preference giving a nod to conciseness, reducing verbosity in tech writing.

To be or not to be: Boolean conversion and minification

Welcome to the world of boolean coercion. The ! in front of a function immediately executed not only changes the function return value to boolean but also flips undefined to true. Echoing the principle behind the !!value negation pattern widely used for truthy/falsy checks.

Don't beat about the byte: Saving bytes and safe execution

With ! stepping up in the minification game, every byte is significant. It also guarantees the function will execute even in concatenated scripts, upholding the order of module execution.

Visualization

Consider the ! as the fuse in a rocket launcher:

!function() {
    // Ready for launch 🚀
}();


Once the fuse (!) is lit, there's no stopping the rocket from launching:

Before: [🚀❌] // The rocket (function) is primed but grounded

After:  [🚀✅] // The fuse has been lit, there's no stopping the rocket now! 🚀

The ! represents an igniting spark, invoking the function immediately.

Exclamation-Bang! Know when to use it

• Swifty Negation: ! flips undefined (or some other falsy return) into true. Efficiently store negated results.
• Concatenation conundrum: When it's about merging files, ! stars as the leading role to ensure no nasty surprises in the execution order, bolstering the modular code approach.
• Legibility vs efficiency: (function(){})() plays it cool with the newbies, but !function(){}() is the compact and efficient choice for seasoned devs.

!(function(){})() vs. (function(){})(): How to choose?

• Shrinking down: With !, you get the same work done while using fewer bytes, that's some serious downsizing for production environments.
• Syntax Error showdown: To avoid dealing with a SyntaxError, always wrap the function in proper parentheses or use !.
• Arrange in order: When scripts are brought together during a build process, using ! will ensure an expected execution order, preventing concatenation errors.

Taking style notes

Coding style guides may dictate using !. To embrace or shy away from ! should be gauged on the "readability vs. efficiency" scale as per team traditions. Resources like the Airbnb JavaScript guide provide key insights.