JavaScript Bang "!" Functions Vs Leading Semi-Colon ";" IIFEs

Airbnd suggests I do this: !function() { // ... }(); Because: This ensures that if a malformed module forgets to include a final semicolon there aren't errors in production w

Solution 1:

You always have IEFEs there. Whether you wrap them in parenthesis or prefix them with a ! is your choice, and doesn't make a difference. You need either one to force the function to be parsed as an expression. See javascript function leading bang ! syntax for details.

Whether you prefix that whole construct with a ; to prevent errors from concatenation with poor-written scripts (What does the leading semicolon in JavaScript libraries do?) is totally unrelated. You can mix the patterns as you want:

 !function(){…}() // not safe for arbitrary concatenation
 (function(){…}()) // not safe for arbitrary concatenation either
;!function(){…}()
;(function(){…}())

However, there is one case of concatenation where () vs ! does make a difference: if two scripts are concatenated so that there is a newline in between, and the former does not end in a semicolon. This does allow automatic semicolon insertion to jump in - when the next line does begin with a bang!

1 + 2             // script A
!function(){…}(); // script B
// works!

1 + 2              // script A
(function(){…}()); // script B
// runtime error: "2 is not a function" (or whatever the previous line ends in)

We learn: Always end your script with a semicolon. Use intelligent concatenation. Start your script with a semicolon if you need to be safe against dumb concatenation.

Solution 2:

The ! has actually nothing to with protecting against missing semicolons when concatenating files. It's often used to enforce that the following function definition is evaluated as expression, not as declaration.

A leading semicolon on the other hand ends any "open" expression statement that might come before it.

But if you follow the explanation, it seems like ! would suffice if it's the first character in the line. Apparently JS would add a semicolon to the previous due to ASI. However, that seems to be a more fragile approach, since you might not be in total control over how the modules are concatenated. Using a semicolon is definitely "safer".

Solution 3:

Actually this works too:

;!function() {
  // ...
}();

!function(){}()!function(){}(); // => SyntaxError: Unexpected token !
!function(){}();!function(){}(); // => true
(function(){}());!function(){}(); // => true
!function(){}();;!function(){}(); // => true
(function(){}());;!function(){}(); // => true

So IIFEs are no better than bang functions, as I absentmindedly-assumed in the last paragraph of my question. Regardless of whether you use a bang function or an IIFE, a leading semi-colon before them is a good defensive technique.