Why Am I Getting 'Uncaught SyntaxError: Unexpected token ILLEGAL' in My JavaScript Code?

No Visible Cause for "Unexpected Token ILLEGAL"

Upon encountering the JavaScript error "Uncaught SyntaxError: Unexpected token ILLEGAL," the initial reaction may be a lack of discernible cause, especially for a seemingly straightforward code like "var foo = 'bar';".

The Error's Source

When JavaScript code is parsed, it breaks down into "tokens," and any token that cannot be categorized into the four basic types receives the "ILLEGAL" label. This error can also arise from misplaced characters or missing syntactic elements, such as stray @ symbols, unbalanced braces, improper quoting, and so on.

Invisible Illegal Character

In cases where the syntax appears valid, the culprit may be an invisible character: the Unicode Zero-width space (ZWSP, or U 200B), denoted as ​. This character commonly causes the "Unexpected token ILLEGAL" error, originating from sources like jsfiddle, which has been known to embed ZWSPs to control word-wrapping.

Identifying the Invisible Character

To spot the ZWSP, enable the display of invisible characters in your editor. In Vim, for instance, it shows as . Online debugging tools like jsbin and CodePen highlight it as a red dot.

Related Issues

ZWSP serves a legitimate purpose, enabling precise line-wrapping control. However, its presence can also introduce other problems, such as:

• Empty strings that are not truly empty due to hidden ZWSPs
• Unexpected whitespace on web pages
• Incorrect styling due to CSS parsing issues
• Misinterpreted encoding leading to character display issues

ECMAScript Specification

While the ECMAScript Specification mentions similar whitespace characters, it lacks explicit mention of ZWSP. However, the reference to "space separators" in the specification suggests that ZWSP should, in fact, be considered whitespace. Nevertheless, current implementations treat it as an unexpected token.

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