Polyfilling is also some kind of "roll-your-own X".

https://en.wikipedia.org/wiki/Polyfill_(programming)

In web programming, it might sometimes be useful to write your own polyfill when you need to add a single feature for backward-compatibility reasons. Well, a third-party polyfill library that provides that missing feature perfectly might be available. However, the library is too big (with respect to file size) or adds too much complexity during the build process (by pulling in lots of other dependencies).