JavaScript has a number of reserved keywords. These words come in three types: JavaScript reserved keywords, future reserved words, and words to avoid.

JavaScript Keywords
break	false	in	this	void
continue	for	new	true	while
delete	function	null	typeof	with
else	if	return	var

 

JavaScript Future Keywords
case	debugger	export	super
catch	default	extends	switch
class	do	finally	throw
const	enum	import	try

The words to avoid are any that are already the names of intrinsic JavaScript objects or functions. Words like String or parseInt are included in this.

Using any of the keywords from the first two categories causes a compilation error when your script is first loaded. Using a reserved word from the third set can cause odd behavior problems if you attempt to use both your variable and the original entity of the same name in the same script. For example, the following script does not do quite what you think it should:

var String;
var text = new String("This is a string object");

In this case, you get an error saying that String is not an object. Many cases of using a pre-existing identifier aren't this obvious.