Spaces, new lines, tabs, semicolons

Tip: Recall "Rules of Automatic Semicolon Insertion" and "The five restricted productions"

Answer: alert with "undefined" text.

Explanation: "return" belongs to "The five restricted productions". The five restricted productions are return, throw, break, continue, and post-increment/decrement. JavaScript interpreter will automatically insert semicolon after "return" like:

 function a(){
	return;
	1;
 }
 alert (a());

Read it for details:
wikipedia.org: Whitespace and semicolons
pdf document: ECMAScript® 2015 Language Specification