Why Am I Getting an 'Unexpected Token ILLEGAL' Error Despite Apparently Correct Code?

An Invisible Cause for "Unexpected Token ILLEGAL" Error

The Syntax Error

When code is broken down into elements called "tokens" during interpretation, those that cannot be categorized get labeled as "ILLEGAL." This error can also occur due to rogue characters or misplaced symbols.

The Invisible Character

While your code may appear simple, an invisible Unicode character called Zero-width space character (ZWSP) could be lurking after the semicolon. This character can cause the Unexpected token ILLEGAL JavaScript syntax error.

The Origin of ZWSP

Tools like jsfiddle often introduce ZWSP to control word-wrapping. While newer versions of jsfiddle and Chrome developer tools have addressed this issue, it's still worth checking for it.

How to Spot ZWSP

To verify the presence of ZWSP, use an editor that displays invisible characters (e.g., Vim). Online debugging tools like jsbin and CodePen.io also highlight ZWSP as a red dot.

Related Issues

ZWSP can also cause:

• Extra whitespace in HTML
• Styling errors in CSS due to unrecognized characters
• False positives in empty string checks

The ECMAScript Specification

The ECMAScript Specification lists similar characters (U 200C and U 200D) as valid IdentifierParts outside of certain contexts. However, U 200B is not explicitly mentioned. Implementations treat it as an unexpected token, leading to the syntax error.

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