Due to their common use in websites, JavaScript files are not detected by traditional Anti-Virus software. Scanning JavaScript files while browsing is virtually impossible due to its performance impact and probable high false positive rate, due to most sites using obfuscated JavaScript as a benign method to protect their intellectual property. To detect such attacks, the contextual data is needed.

http://blog.checkpoint.com/2017/05/01/getting-emails-blocked-dont-forget-semicolon/