A released research made by the firm Akamai Technology has shown that an average of 25~26% of javascript malwares are likely undetectable by anti-malwares due to “Obfuscation” techniques.
Obfuscation consists of turning a readable, clear source code into an unreadable, confusing and un-debuggable code, to complicate analysis and detection tasks.
Except malicious malwares which are the most use-case for obfuscation, the research shows that this technique is also used at about 0.5% on common non-malicious websites which want to make it harder for the public to read their code in particular when it is about code functionality.
source : https://www.akamai.com/blog/security/over-25-percent-of-malicious-javascript-is-being-obfuscated