What does the leading semicolon in JavaScript libraries do?

Question

In several JavaScript libraries I saw this notation at the very beginning:

/**
 * Library XYZ
 */
;(function () {
  // ... and so on

While

Answer 1

A one-line answer is to safely concatenate multiple JavaScript files. Using a semicolon does not raise an issue.

Suppose you have multiple functions:

IIFE 1

(function(){
  // The rest of the code
})(); // Note it is an IIFE

IIFE 2

(function(){
   // The rest of the code
})(); // Note it is also an IIFE

On concatenation it may look like:

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

But if you add a semicolon before the function it will look like:

;(function(){})();(function(){})()

So by adding a ;, it takes care if any expression is not properly terminated.

Example 2

Assume you have a JavaScript file with a variable:

var someVar = "myVar"

Another JavaScript file with some function:

(function(){})()

Now on concatenation it will look like

var someVar = "myVar"(function(){})() // It may give rise to an error

With a semi-colon, it will look like:

var someVar = "myVar";(function(){})()