The dynamic features of the JavaScript language not only promote various means for users to interact with websites through Web browsers, but also pose serious security threats to both users and websites. On top of this, obfuscation has become a popular technique among malicious JavaScript code that tries to hide its malicious purpose and to evade the detection of anti-virus software. To defend against obfuscated malicious JavaScript code, in this paper we propose a mostly static approach called JStill. JStill captures some essential characteristics of obfuscated malicious code by function invocation based analysis. It also leverages the combination of static analysis and lightweight runtime inspection so that it can not only detect, but also prevent the execution of the obfuscated malicious JavaScript code in browsers. Our evaluation based on real-world malicious JavaScript samples as well as Alexa top 50,000 websites demonstrates high detection accuracy (all in our experiment) and low false positives of JStill. Meanwhile, JStill only incurs negligible performance overhead, making it a practical solution to preventing obfuscated malicious JavaScript code.