Challenge 3: Diagnose a Missing Checkout — Possible Solution
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THE SCENARIO
-------------
A workflow's first step is run: npm install (no actions/checkout
beforehand), and it fails immediately with an error about a missing
package.json.

WHY THIS HAPPENS
-----------------
Per Chapter 2's core point about job isolation — reinforced directly in
this chapter's own tip box — every job runs on a completely fresh
runner that starts with an EMPTY filesystem. There's no automatic
mechanism that puts the repository's code onto that runner; it has to
be explicitly fetched via actions/checkout as an actual step in the
workflow. Without that step ever running, the runner genuinely has no
files from the repository at all — not package.json, not any source
code, nothing.

When npm install then runs as the very first step, it looks in the
current working directory for a package.json to read (to know what
dependencies to install) — and finds nothing there at all, because the
runner's filesystem is still in its pristine, empty starting state.
The resulting error ("missing package.json" or similar) isn't a bug in
npm install itself; it's the direct, expected consequence of trying to
run a command that depends on repository files existing, on a machine
that was never given those files in the first place.

WHY THIS ISN'T ABOUT npm install BEING WRONG
--------------------------------------------------
This chapter is explicit that this exact failure mode is "not because
those commands are wrong, but because there's simply no repository
present to run them against." The fix has nothing to do with how
npm install is invoked or configured — the missing piece is an entirely
separate, prior step: adding actions/checkout@v4 (or the current
version) as the FIRST step in the job, before npm install or any other
command that assumes the repo's files are already present.

THE FIX
--------
steps:
  - uses: actions/checkout@v4   # <-- the missing, required first step
  - run: npm install
  - run: npm test

Adding this one line resolves the failure entirely, since the runner
now actually has the repository's files (including package.json)
present before npm install ever tries to read them.