Semicolon before self-invoking function?

Self-invoking functions are surrounded by parentheses, and in JavaScript parentheses are overloaded to mean

1. Grouping expressions to override precedence: (x + y) * z
2. Function application : f()

Putting a semicolon before the function prevents the function from becoming an argument to whatever precedes it when the parentheses become confused with function application.

Consider

var x = 42

(function () { ... })()

is the same as

var x = 42(function () { ... })()

but

var x = 42

;

(function () { ... })()

is the same as

var x = 42;

(function () { ... })()

I write all JavaScript in a semicolon-free style. When writing without semicolons at the end of every line, due to Automatic Semicolon Insertion (ASI), there are a few special cases that can "be confusing" at first:

1. Starting a top-level expression with an operator, a ( (open parenthesis) in this case, which like most other operators, can continue the previous expression and thus suppresses the "automatic insertion of a semicolon". (This generally only occurs when using a self invoking function.)

2. Just kidding about #2: there isn't one! (Learn only one rule and you too can enjoy life without extra semicolons ;-)

Since I write in a semicolon-free style I thus always write it as (where the function-expression can naturally span multiple lines):

;(FunctionExpression)()

In my case it isn't about "safety" or trying to "catch an error" (honestly, if your style is to use semi-colons and you forget a semi-colon, then you've already created the error elsewhere and writing a ; at the start for "safety" is hogwash). No; in my case it is done for consistency with knowledge of my chosen style and "knowing" that starting a line with an operator can continue an expression from a previous line.

See JavaScript: Semicolon Insertion (Everything You Need To Know) for the details (it is by far the best article I have seen on the subject).

Happy coding.

If you concatenate two files with self-invoking functions together that look like this:

File A:

(function(){...A...})()

File B:

(function(){...B...})()

File A+B:

(function(){...A...})()(function(){...B...})()

You have two statements without separator. This happens when you cat files together and then minify them.

Now the author of file B puts a semicolon in front:

File B2:

;(function(){...B2...})()

And you'll get a working script:

(function(){...A...})();(function(){...B2...})()