Challenge 2: Set Up an Incremental Build — Possible Solution
====================================================================

// tsconfig.json

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "commonjs",
    "strict": true,
    "outDir": "./dist",
    "rootDir": "./src",
    "incremental": true,
    "tsBuildInfoFile": "./.cache/tsbuildinfo.json"
  },
  "include": ["src/**/*.ts"]
}

// .gitignore should include the cache directory:
//   .cache/

WHAT CHANGES ON THE SECOND tsc RUN
-----------------------------------
- FIRST RUN: tsc has no prior information, so it type-checks every file in
  the project from scratch. It then writes a build-info file (here,
  ./.cache/tsbuildinfo.json) recording, for every source file, things like
  its content hash, which other files it depends on, and a summary of what
  was already verified.

- SECOND RUN (with no changes): tsc reads the existing .tsbuildinfo file,
  compares each file's current content hash against what's recorded, and
  since nothing changed, it can skip re-checking files it already knows
  are unchanged and unaffected — the run is dramatically faster than the
  first, often by an order of magnitude on a large project.

- SECOND RUN (after editing one file): tsc uses the dependency information
  in .tsbuildinfo to figure out not just that the edited file changed, but
  which OTHER files import it (and therefore might be affected by the
  change). Only that edited file plus its dependents get re-checked —
  files completely unrelated to the change are skipped entirely.

- Without "incremental": true, every single `tsc` invocation re-checks the
  entire project from a cold start every time, regardless of how small the
  actual change was — this is the behavior incremental builds are
  specifically designed to avoid.

WHY THIS WORKS
--------------
- "incremental": true is the switch that tells tsc to write and consult a
  build-info cache at all — without it, "tsBuildInfoFile" has nothing to do.
- Pointing tsBuildInfoFile at a dedicated cache directory (rather than
  leaving it at the default location alongside compiled output) keeps the
  cache file out of source control and out of the way of the actual build
  artifacts in "outDir".
- This is the single-line change that gives the biggest, easiest
  performance win mentioned in the chapter — no type simplification or
  code restructuring required, just telling the compiler to remember what
  it already checked.