The Hermes Explorer is similar in spirit to the Godbolt Compiler Explorer, but for the Hermes JavaScript engine. It allows people to run the Hermes Compiler and Hermes JavaScript engine in their browser window, and observe and study the compiler output.

You can try it here.

It runs an (almost) unmodified Hermes executable compiled to asm.js in the browser and supports exactly the same options. Some interesting options to try:

• -O -dump-bytecode : dump bytecode with optimizations
• -O -dump-ir: dump the SSA internal representation with optimizations
• -dump-ast: dump the parse tree
• -O: compile the code with optimizations and run it.

Please feel free to enter --help and explore all other command line options.

Q: Is it safe to run arbitrary JavaScript in the Hermes Explorer?

A: Yes. Since Hermes itself is compiled to asm.js, it is technically running in a sandbox.

Q: But why would I want to do this? Isn't v8 enough?

A: While Hermes is not meant to run in a browser window, having a JavaScript sandbox presents some interesting possibilities. More importantly, this is meant to demonstrate that Hermes is lightweight and embeddable.