Week 3 Session: The shim that almost works

Run real programs against a fake runtime and find the exact line where impersonation stops paying

8 March 2027

Pick a Node, Bun or Deno program — this week needs a JavaScript runtime target, so if your assignment target is native, borrow something else for the hour.

Suggestions, chosen because they break in different places:

• a CLI that only touches fs and path
• one that shells out — a wrapper around git, ffmpeg or similar
• one that uses os, process or native addons
• a test runner, which usually does all three

Get it running against a browser-side Node shim, and record where the work went. You are looking for the boundary, not a working program.

The useful finding is not “it broke”. It is which kind of thing broke:

• an API whose work is pure JavaScript — falls out, almost free
• an API that wants the operating system — a shim can fake it, badly or well
• an API that needs to execute something — there is nothing to execute

A shell is a good demonstration of the first two: a shell written in JavaScript will give you cd, pipelines and job control quite happily. Ask it for git and the illusion ends, because git is not an interface. It is a binary.

Bring the line you found. Weeks 4 and 5 are two different answers to it.