what does a semicolon before a function mean?

Question

I'm looking at an open source project, and see something like this :

;(function(){
    // codes here
})()

I'd like to know whether the semicolon there has a special meaning?

Answer 1

This is because the ASI (Automatic Semicolon Insertion) allows you to avoid semicolons.

For example, you can write this kind of code with no error:

var a = 1
a.fn = function() {
    console.log(a)
}

See? Not a single semicolon.

However, there are cases where the semicolon isn't inserted. Basically, in real projects, there is one case where it isn't: when the next line starts with a parenthesis.

The javascript parser will take the next line as an argument and not automatically add a semicolon.

Example:

var a = 1
(function() {})()
// The javascript parser will interpret this as "var a = 1(function() {})()", leading to a syntax error

To avoid this, there are several ways:

• Add a semicolon at the beginning of a line starting with a parenthesis (which was done in the code you show)

• Use the following structure:

  !function() {}()

Answer 2

JavaScript has automatic semicolon insertion (see section 7.9 in ECMAScript Language Specification):

There are three basic rules of semicolon insertion:

1. When, as the program is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
   • The offending token is separated from the previous token by at least one LineTerminator.
   • The offending token is }.
2. When, as the program is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single complete ECMAScript Program, then a semicolon is automatically inserted at the end of the input stream.

Usually you can omit the last semicolon in a JavaScript file (second rule). If your application creates JavaScript code by merging several files this will result in a syntax error. Since ; itself is the empty statement you can use it to prevent such syntax errors.

Answer 3

A very good explanation can be found here:

https://mislav.net/2010/05/semicolons/
(see paragraph "The only real pitfall when coding without semicolons")

var x = y
(a == b).print()

is evaluated as

var x = y(a == b).print()

Bottom line, it's a good practice to put a semicolon before every row that starts with a "(" character