  Open source licenses create a legal framework that plays a crucial role in
the widespread adoption of open source projects. Without a license, any source
code available on the internet could not be openly (re)distributed. Although
recent studies provide evidence that most popular open source projects have a
license, developers might lack confidence or expertise when they need to
combine software licenses, leading to a mistaken project license
unification.This license usage is challenged by the high degree of reuse that
occurs in the heart of modern software development practices, in which
third-party libraries and frameworks are easily and quickly integrated into a
software codebase.This scenario creates what we call "multi-licensed" projects,
which happens when one project has components that are licensed under more than
one license. Although these components exist at the file-level, they naturally
impact licensing decisions at the project-level. In this paper, we conducted a
mix-method study to shed some light on these questions. We started by parsing
1,426,263 (source code and non-source code) files available on 1,552 JavaScript
projects, looking for license information. Among these projects, we observed
that 947 projects (61%) employ more than one license. On average, there are 4.7
licenses per studied project (max: 256). Among the reasons for multi-licensing
is to incorporate the source code of third-party libraries into the project's
codebase. When doing so, we observed that 373 of the multi-licensed projects
introduced at least one license incompatibility issue. We also surveyed with 83
maintainers of these projects aimed to cross-validate our findings. We observed
that 63% of the surveyed maintainers are not aware of the multi-licensing
implications. For those that are aware, they adopt multiple licenses mostly to
conform with third-party libraries' licenses.
