WebAssembly WAT samples

Some sample WebAssembly Text programs.

WebAssembly defines a portable binary code format and a corresponding text format for executable programs as well as software interfaces for facilitating interactions between such programs and their host environment.

The text format (WAT) is, essentially, an assembly language. While most people will not write WAT directly (just as most programmers don't write assembly code directly), familiarity this format is important if you seek a deep understanding of WASM mechanisms and related toolchains.

How to run these samples

Unless otherwise stated, each sample consists of a single .wat file and an accompanying test.js file, housed in their own directory. When the setup is more complicated, the directory's wwn README will have additional information.

To run a single sample, cd into its directory, and start by compiling the WAT file to WASM with watgo, wasm-tools or some other WASM toolchain. Let's use the stack example for demonstration:

$ cd stack
$ watgo parse stack.wat -o stack.wasm

This creates a stack.wasm binary, which test.js expects to find in its own directory. To load and test the WASM, you'll need a recent Node.js installed, and run:

$ node test.js

This will typically emit some output and finish successfully if all went well; if the loading failed or the loaded WASM behaves unexpectedly, the test.js script will report an error.

An alternative way to run a single sample is using the run-tests.js runner, mentioned in the next section. From the root directory of the project, run:

$ node run-tests.js stack

And it will automatically do the steps described above.

Running all tests

The run-tests.js script is a test runner for the entire project:

$ node run-tests.js

It should be run from the root directory of the project. This command will build all .wat files in all directories and run the corresponding test.js files, summarizing the results.

WASI documentation

Useful sources of documentation about WASI host calls:

• Preview 1 ABI
• wasi-libc sources; in particular, the wasi/api.h header