Why Does My JavaScript Code Throw an 'Unexpected token ILLEGAL' Error?

"Unexpected token ILLEGAL": A Hidden Culprit

Despite its straightforward appearance, the code that caused the "Unexpected token ILLEGAL" error contains an invisible character that is the actual culprit. This invisible character is the Unicode U 200B Zero-width space character (ZWSP).

Invisible Character Origin

The source of the ZWSP character is often unknown, but it is commonly found in code pasted from jsfiddle or the Chrome developer tools. In jsfiddle, the character is used to control word-wrapping.

Identifying the Invisible Character

Although invisible, the ZWSP character can be detected by enabling the display of invisible characters in your code editor or using online debugging tools. Editors like Vim display the character as , while tools like jsbin and CodePen.io show it as a red dot.

Effects and Related Problems

The ZWSP character can cause various problems, including:

• JavaScript syntax errors
• Invisible whitespace or extra whitespace on web pages
• Display of the character as non-whitespace characters if the web page encoding is not recognized as UTF-8
• Improper parsing of CSS code leading to styling issues

ECMAScript Specification

The ECMAScript Specification does not explicitly mention the U 200B character as an unexpected token. However, it suggests that similar characters (U 200C and U 200D) should be treated as part of identifiers when not within certain code elements. The behavior of implementations (Chrome and Firefox) in treating the ZWSP as an unexpected token appears inconsistent with the specification.

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