Poorly written Javascript is not suitable for large web applications. This goes for any language. Competent Javascript developers will have many files that are concatenated into just one or two files as part of their build environment. Competent Javascript developers will use code quality tools such as jslint or jshint to find potential problems.
It’s too bad that you inherited a poorly written Javascript project, but that doesn’t mean the language you’re most familiar with was the best language for the job. This is a common response. “I know X, let’s use X!!!”