var aString = 'this is a string with a // in it '.'and a concatenation';
The syntax highlighter is treating the // in the string as a comment start, but only until the end of the string. This results in everything between the // and the next ' character to be formatted grey as a comment, which is incorrect.
Question is old but writing for reference that may be helpful.
The syntax error regarding comment start may have been caused by another syntax error. I have tried this in Netbeans IDE and didn't get any syntax error:
const aString = 'this is a string with a // in it '.'and a concatenation';
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