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

The best answer was actually given in the question, so I will just write that down here for clarity:

The leading ; in front of immediately-invoked function expressions is there to prevent errors when appending the file during concatenation to a file containing an expression not properly terminated with a ;.

Best practice is to terminate your expressions with semicolons, but also use the leading semicolon as a safeguard.