A libdef is a special file that informs Flow about the type signature of some specific third-party module or package of modules that your application uses. If you’re familiar with languages that have header files (like
C++), you can think of libdefs as a similar concept.
These special files use the same
.js extension as normal JS code, but they are placed in a directory called
flow-typed in the root directory of your project. Placement in this directory tells Flow to interpret them as libdefs rather than normal JS files.
NOTE: Using the
/flow-typeddirectory for libdefs is a convention that enables Flow to JustWork™ out of the box and encourages consistency across projects that use Flow, but it is also possible to explicitly configure Flow to look elsewhere for libdefs using the
[libs]section of your
Try to provide a libdef for each third-party library your project uses
If a third-party library that has no type information is used by your project, Flow will treat it like any other untyped dependency and mark all of its exports as
any. Interestingly, this is the only place that Flow will implicitly inject
any into your program.
Because of this behavior, it is a best practice to find or write libdefs for as many of the third-party libraries that you use as you can. We recommend checking out the
flow-typed tool and repository , which helps you quickly find and install pre-existing libdefs for your third-party dependencies.
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