As JavaScript is becoming more and more popular, JavaScript programs are valuable assets to many companies. However, the source code of JavaScript programs can be easily obtained and plagiarism of JavaScript programs is a serious threat to the industry. There are techniques like code obfuscation and watermarking which can make the source code of a program difficult to understand by humans and prove the ownership of the program. However, code obfuscation cannot avoid the source code being copied and a watermark can be defaced. In this paper, we use a relatively new technique, software birthmark, to help detect code theft of JavaScript programs. A birthmark is a unique characteristic a program possesses that can be used to identify the program. We extend two recent birthmark systems that extract the birthmark of a software from the run-time heap. We propose a redesigned system with improved robustness and performed extensive experiments to justify the effectiveness and robustness of it. Our evaluation based on 200 large-scale websites showed that our birthmark system exhibits 100% accuracy. We remark that it is solid and ready for practical use.