How to (re)build the Javascript from the C sources

You will not need to do this for a fresh checkout, since the project policy is to include the generated javascript as a checked-in file.

Ingredients

Within the js-nacl directory,

import.py massages an unpacked nacl tarball, extracting just the reference implementations of the various algorithms. (See its origin in Brian Warner's minimal branch of pynacl.)
keys.c extends core nacl with a pynacl-compatible implementation of crypto_sign_keypair_from_raw_sk, the foundation for nacl.crypto_sign_keypair_from_seed.
nacl_cooked.js is the high-level Javascript interface to the low-level code.
nacl_cooked_prefix.js and nacl_cooked_suffix.js are wrapped around nacl_cooked.js and nacl_raw.js to create lib/nacl_factory.js.
nacl_randombytes_emscripten.js is infrastructure for making the Emscripten-compiled library parameterized in its source of randomness.
test/runner.html runs the Mocha tests in the browser.
test/tests.js is the source code for the test cases themselves.
benchmark.html and benchmark.js are trivial speed measurements for running in the browser.

Follow the instructions from the Emscripten tutorial to get Emscripten ready to run.

Once emcc is on your $PATH somewhere, use the js-nacl Makefile.

To rebuild everything:

make veryclean all

This will completely remove any compilation products, and recompile everything.

Other Makefile targets:

make or make test: builds the library if necessary and then runs the test suite using node.js.
make clean: removes generated Javascript, but does not remove the unpacked and pre-processed nacl tarball contents.
make veryclean: as make clean, but also removes the subnacl directory and the contents of the nacl tarball.
make all: performs all the build steps.

If you for some reason need to use a different python than python, set the PYTHON makefile variable; for example,

make PYTHON=python2.7

Similarly, set EMCC to the path to your emcc binary if it's not on your $PATH.

The build products will be in