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.
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.
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.
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.
ZWSP can also cause:
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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