Deno FFI is an API in the Deno namespace that allows the Deno JavaScript / TypeScript runtime to access data and call functions from native dynamic libraries. On Linux a native library would be something like libfoo.so, while on Windows it would be foo.dll. Note that these are not executables. If running executables is required, then the Deno.run or Deno.spawn APIs can be used.

Deno FFI works with native libraries that use the C API. As an example, Deno FFI cannot call Rust libraries that do not expose C APIs using the external "C" declaration, at least not reliably or without considerable pain.

Using Deno FFI is made up of two steps:

  1. Opening (loading) a native library with some symbol declarations.
  2. Using the symbols returned by the open API to call into native code.
// Open a native library
const lib = Deno.dlopen(
    // Declare symbols
    method: {
      parameters: ["u8"],
      result: "pointer",
  } as const,

// Use symbols
const resultPointer = lib.symbols.method(35);

The Deno.dlopen API requires both the --unstable flag (meaning that this API is still in flux and there may be API breaks with even Deno patch releases) and the --allow-ffi or --allow-all / -A flags. If one or both flags are not given, the opening of the library will throw an error. The opening will also throw an error if a declared symbol name is not found in the library. Symbol types cannot be and are not checked by the open function, so the type declarations and their validity are left entirely up to the user.

Calling the symbols returned by Deno.dlopen does not require the --allow-ffi flag. As such, it is possible to revoke FFI permissions after opening a library and still keep using the symbols.

Note that the symbols declaration is set type-wise readonly using as const. The reason for this is that changing the symbol declarations may cause undefined behaviour and thus the typings for Deno.dlopen expect readonly declarations. If non-readonly declarations are passed in, the type inference for the symbols will not work properly.