Uncaught Syntaxerror: Unexpected Token Illegal

No visible cause for Unexpected token ILLEGAL

The error

When code is parsed by the JavaScript interpreter, it gets broken into pieces called "tokens". When a token cannot be classified into one of the four basic token types, it gets labelled "ILLEGAL" on most implementations, and this error is thrown.

The same error is raised if, for example, you try to run a js file with a rogue @ character, a misplaced curly brace, bracket, "smart quotes", single quotes not enclosed properly (e.g. this.run('dev1)) and so on.

A lot of different situations can cause this error. But if you don't have any obvious syntax error or illegal character, it may be caused by an invisible illegal character. That's what this answer is about.

But I can't see anything illegal!

There is an invisible character in the code, right after the semicolon. It's the Unicode U+200B Zero-width space character (a.k.a. ZWSP, HTML entity ). That character is known to cause the Unexpected token ILLEGAL JavaScript syntax error.

And where did it come from?

I can't tell for sure, but my bet is on jsfiddle. If you paste code from there, it's very likely to include one or more U+200B characters. It seems the tool uses that character to control word-wrapping on long strings.

UPDATE 2013-01-07
After the latest jsfiddle update, it's now showing the character as a red dot like codepen does. Apparently, it's also not inserting U+200B characters on its own anymore, so this problem should be less frequent from now on.
UPDATE 2015-03-17
Vagrant appears to sometimes cause this issue as well, due to a bug in VirtualBox. The solution, as per this blog post is to set sendfile off; in your nginx config, or EnableSendfile Off if you use Apache.

It's also been reported that code pasted from the Chrome developer tools may include that character, but I was unable to reproduce that with the current version (22.0.1229.79 on OSX).

How can I spot it?

The character is invisible, do how do we know it's there? You can ask your editor to show invisible characters. Most text editors have this feature. Vim, for example, displays them by default, and the ZWSP shows as <u200b>. You can also debug it online: jsbin displays the character as a red dot on its code panes (but seems to remove it after saving and reloading the page). CodePen.io also displays it as a dot, and keeps it even after saving.

The ECMAScript Specification

I couldn't find any mention to that specific character on the ECMAScript Specification (versions 3 and 5.1). The current version mentions similar characters (U+200C and U+200D) on Section 7.1, which says they should be treated as IdentifierParts when "outside of comments, string literals, and regular expression literals". Those characters may, for example, be part of a variable name (and var x\u200c; indeed works).

Section 7.2 lists the valid White space characters (such as tab, space, no-break space, etc.), and vaguely mentions that any other Unicode “space separator” (category “Zs”) should be treated as white space. I'm probably not the best person to discuss the specs in this regard, but it seems to me that U+200B should be considered white space according to that, when in fact the implementations (at least Chrome and Firefox) appear to treat them as an unexpected token (or part of one), causing the syntax error.

Node JS: Unexpected token ILLEGAL

Syntax you trying to use is the part of new ES2015 standard. Make sure you use node version which supports Template Strings

I suggest at least node 4.2.3 because it's LTS version and no iojs because it's merged back to node.

JavaScript Uncaught SyntaxError: Unexpected token ILLEGAL

In the rendered JavaScript in the HTML attribute text, the string you have in path2 won't be in quotes, it'll look like this:

onerror='nofind(this,images/1s.jpg);'

One solution is to put it in quotes:

var tmp = "<img src='"+path1+"' onerror='nofind(this,\""+path2+"\");' /> ";
// Change is here -----------------------------------^^---------^^

...which renders like this:

onerror='nofind(this,"images/1s.jpg");'

A better solution, though, would be not to use an onXyz attribute at all. There's no reason to. Instead, use modern event handling:

function onc(){
    var path1= 'images/1h.jpg';//Does not exist
    var path2= 'images/1s.jpg';//exist
    var img = $("img").on("error", function() {
                  nofind(this, path2);
              }).attr("src", path1);
    $('div').html(img);
}

Note that we hook the error event before setting the src attribute. This may not be strictly necessary with error depending on the reason for the error, but it's a good habit to get into and it can matter quite a lot for the related load event. (It's a common misconception that you can hook the load event after setting src; it works most of the time, but may not if the browser has the image cached and the cache settings for it allow the browser to reuse it without revalidating it, in which case the browser may fire the load event as soon as src is set and, not seeing any JavaScript handlers, doesn't queue them to run when the JavaScript thread is next free to run handlers. [There is only one main JavaScript UI thread, but browsers are multi-threaded.])