Ask HN: How do you keep third party code size to a minimum?

At my job we have a series of single page applications to handle a variety of things. As time goes by, I'm seeing more and more third-party libraries being imported to help out with new features and bug fixes. The problem is, each library has it's own size, and our users are downloading them. We never use the entire library, and usually only use a small fraction of it. What are some best practices you guys do to keep your codebase as small as possible?