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Build a Scaffold Codegen CLI

Scaffold Codegen CLI — a complete walkthrough of project scaffolding, covering template loader, variable prompt, file renderer, and testing.

scaffold_codegen_cli/
├── src/index.ts
├── src/template-loader.ts
├── src/variable-prompt.ts
├── tests/scaffold_codegen_cli.test.ts
└── package.json

// Template loader: project scaffolding core
export function templateloader(input: TemplateloaderInput): TemplateloaderResult {
  // Build the template loader that anchors project scaffolding and defines the core data shape.
  return processTemplateloader(input);
}

// Variable prompt — handles project scaffolding Add the variable prompt so
export class Variableprompt {
  constructor(private opts: VariablepromptOptions) {}
  async run(ctx: Context): Promise<VariablepromptOutput> {
    return this.execute(ctx);
  }
}

// File renderer: project scaffolding stage
export const filerenderer = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

// Hook runner for project scaffolding
export async function hookrunner(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

// Output writer: project scaffolding finalization
export function outputwriter(state: State): Result {
  validate(state);
  return finalize(state);
}

Project overview

This project builds a project scaffolding system whose reason for existing is generating a runnable project skeleton from a template so new services start consistent. The temptation in project scaffolding is to bolt the interesting part onto a working prototype and discover the hard cases in production; this build inverts that order and treats a template that hardcodes a path and generates files outside the target directory as the first thing designed, not the last.

You will implement Template loader, Variable prompt, File renderer, Hook runner, and Output writer, each as a module with one responsibility and a test that proves it in isolation. The project scaffolding pipeline is then the wiring that composes them, and the integration test is where the composition is shown to survive a template that hardcodes a path and generates files outside the target directory.

The walkthrough is written for a developer who has used a project scaffolding system but has not had to make one honest. It favors explicit data shapes over clever abstractions, and it names the failure each design choice prevents rather than presenting the choice as a style preference.

Each phase ends with something observable — a template loader that returns the right shape, a variable prompt that rejects bad input, an integration test that goes green — so you can stop at any point and still have a project scaffolding system that works for the cases covered so far.

What you will build

The finished project scaffolding system exposes five capabilities, each with a focused test:

• Template loader
• Variable prompt
• File renderer
• Hook runner
• Output writer

Template loader is the foundation that every later stage assumes; Variable prompt is where project scaffolding correctness lives; File renderer handles a template that hardcodes a path and generates files outside the target directory; Hook runner makes the system observable; and Output writer is the graceful-degradation layer that keeps overload from becoming corruption.

The build keeps the project scaffolding logic in ordinary modules you can read in one sitting, and pulls in dependencies only for the parts that are genuinely tedious to hand-roll. That keeps the diff between the tutorial and a real codebase small, and keeps the failure modes visible in code rather than hidden behind a framework.

Architecture and data model

The system is organized around Template loader, Variable prompt, and File renderer, with Hook runner tying them together. Data flows as a template tree of files with placeholder variables, rendered with user-supplied values and post-processed by hooks.

Each component is a module with a pure interface where practical: the same input produces the same output, and side effects are pushed to the edges. That discipline is what makes the phases independently testable, because a test for File renderer does not need to stand up Template loader and Variable prompt to exercise the interesting branch.

The data model is deliberately explicit. A template tree of files with placeholder variables, rendered with user-supplied values and post-processed by hooks Keeping the shape written down — in a schema, a type, or a fixture — means a change to the shape is a visible diff rather than a silent runtime shift, and it means the integration test can assert against a concrete shape instead of guessing.

Configuration and code are kept apart: the configuration says what project scaffolding should do, the modules say how. A reviewer can change a threshold or a policy without reading the implementation, which keeps the risky changes small and lets the diff stand as the review.

The failure path through File renderer is designed before the happy path is finished, because a template that hardcodes a path and generates files outside the target directory is the case that distinguishes a project scaffolding system from a demo. Every component declares what it does when its inputs are wrong, and the integration test asserts each of those declarations.

Prerequisites and dependencies

Have these ready before you install anything, so the project scaffolding setup is not blocked by a missing tool:

• Node.js 20 LTS or later
• Familiarity with project scaffolding fundamentals
• A local environment to run the test suite

The runtime dependencies are chosen to keep the project scaffolding datapath legible:

• commander (^12.1.0) — CLI framework
• ejs (^3.1.10) — Template rendering
• glob (^11.0.0) — Template files

# setup for scaffold-codegen-cli
pnpm init
pnpm add commander ejs glob
pnpm add -D typescript tsx vitest

Versions are pinned to a major range so the project scaffolding install is reproducible; commit the lockfile. The template loader reads its operational parameters from the environment and validates them at startup, so a missing value fails the boot rather than the first project scaffolding request.

Directory structure

Keep each project scaffolding component in its own file so it can be tested without starting the others:

scaffold_codegen_cli/
├── src/index.ts
├── src/template-loader.ts
├── src/variable-prompt.ts
├── tests/scaffold_codegen_cli.test.ts
└── package.json

Phase 1: Template loader

Build the template loader that anchors project scaffolding and defines the core data shape.

// Template loader: project scaffolding core
export function templateloader(input: TemplateloaderInput): TemplateloaderResult {
  // Build the template loader that anchors project scaffolding and defines the core data shape.
  return processTemplateloader(input);
}

After this phase the template loader works in isolation and is covered by a unit test. The template loader defines the data shape that every later stage assumes, so getting it right first means the later phases compile against a stable contract.

Treat the template loader as the single source of truth for the project scaffolding data shape. Validate inputs at the boundary and reject early, so a malformed value never reaches Variable prompt or File renderer where its effect would be harder to localize.

Phase 2: Variable prompt

Add the variable prompt so project scaffolding can generating a runnable project correctly.

// Variable prompt — handles project scaffolding Add the variable prompt so
export class Variableprompt {
  constructor(private opts: VariablepromptOptions) {}
  async run(ctx: Context): Promise<VariablepromptOutput> {
    return this.execute(ctx);
  }
}

After this phase the variable prompt works in isolation and is covered by a unit test. This is where project scaffolding correctness lives: the variable prompt is what makes generating a runnable project skeleton from a template so new services start consistent true, and its test is the proof.

Keep the variable prompt pure where you can. A function that takes an input and returns an output is trivial to test and impossible to mock wrong; a class with hidden state is the opposite. Push side effects to the caller and keep the variable prompt about the transformation.

Phase 3: File renderer

Wire the file renderer to handle a template that hardcodes without losing data.

// File renderer: project scaffolding stage
export const filerenderer = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

After this phase the file renderer works in isolation and is covered by a unit test. The file renderer is the difference between a project scaffolding system that works on the happy path and one that survives a template that hardcodes a path and generates files outside the target directory.

Design the failure before the success. Decide what file renderer does when a template that hardcodes a path and generates files outside the target directory: reject, retry, shed, or degrade — and write that test first. A project scaffolding system whose failure behavior is unspecified will pick one at random under pressure, and it is usually the wrong one.

Phase 4: Hook runner

Integrate the hook runner to make project scaffolding observable and testable end to end.

// Hook runner for project scaffolding
export async function hookrunner(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

After this phase the hook runner works in isolation and is covered by a unit test. Observability is not a finishing touch; it is how you know the project scaffolding system is doing what it claims in production.

Emit one structured event per meaningful action, with enough context to localize a failure without a second round-trip. The hook runner is what turns “something is slow” into “the variable prompt is slow for inputs matching this shape,” which is the difference between an alert and an investigation.

Phase 5: Output writer

Finalize the output writer so the project scaffolding system degrades gracefully under overload.

// Output writer: project scaffolding finalization
export function outputwriter(state: State): Result {
  validate(state);
  return finalize(state);
}

After this phase the output writer works in isolation and is covered by a unit test. The output writer is what keeps overload from becoming corruption: when the project scaffolding system is asked for more than it can do, it degrades in a defined way instead of failing in an undefined one.

Pick a degradation strategy for output writer — shed, queue, or backpressure — and make it explicit in code and in a test. Leaving the behavior to chance is how a project scaffolding traffic spike becomes a data loss incident.

Testing

Each phase has a focused test: template loader, variable prompt, and file renderer are exercised in isolation, then an integration test drives the full project scaffolding pipeline and asserts the a template that hardcodes case is handled.

Start with unit tests that pin each component’s contract: the template loader returns the documented shape, the variable prompt transforms correctly, and the file renderer fails in the documented way. These tests are fast and they localize regressions to the component that broke.

Then add an integration test that composes the components and drives a realistic project scaffolding scenario end to end. Assert the happy path first, then deliberately trigger a template that hardcodes a path and generates files outside the target directory and assert the system degrades rather than corrupts. Because the components are pure where practical, the integration test stays deterministic rather than flaky.

A project scaffolding suite must also prove the negative: bad input is rejected at the boundary, overload is shed rather than queued indefinitely, and a partial failure does not leave the system in an inconsistent state. These are the tests that catch the regressions a happy-path suite never would.

The entire project scaffolding suite runs with no external service; every dependency is either in-process or stubbed, so every case is reproducible in CI. That is what lets the file renderer failure-mode assertions live as tests rather than as a manual checklist that drifts out of date.

# run the project scaffolding test suite
pnpm vitest run

Deployment and operations

Run at least two project scaffolding instances behind a load balancer and share state in Redis. Set resource limits so project scaffolding overload surfaces as backpressure rather than failure.

Run at least two stateless instances behind a load balancer, and move any shared state into a shared store so the project scaffolding behavior is global rather than per instance. Size limits so the system sheds load before it runs out of memory, because a project scaffolding system that OOMs under load fails in a way that is hard to reason about.

Drain on deploy: stop accepting new work, let in-flight project scaffolding operations finish, then exit. Because the system holds no state that cannot be reconstructed, a rollout is a matter of cycling instances one at a time while watching the error rate.

When a dependency is unavailable, decide explicitly whether the project scaffolding system fails closed or degrades. Leaving that choice to chance is how a dependency outage becomes a silent data loss, so make the choice visible in code and in a test.

Security considerations

Validate all project scaffolding inputs at the boundary, reject oversized payloads before buffering them, and keep secrets in the environment rather than the repository.

The project scaffolding system is a choke point, so it is the right place to reject malformed input before it reaches variable prompt or file renderer. Validate the shape and the size at the edge, and let the inner components assume what they receive is well-formed.

Never log secrets or full payloads; log enough to localize a failure without recording data that should not be on disk. Treat any external call as untrusted and validate its response before using it, because a project scaffolding system that trusts its inputs blindly is one malformed response away from corruption.

Keep dependencies pinned and run an audit in CI. A transitive dependency with a known vulnerability is still a vulnerability in your project scaffolding system, and the lockfile is what makes the audit meaningful.

Observability

Emit one structured event per project scaffolding operation with the inputs’ shape, the outcome, and a correlation id. Track error rate and latency per component and alert on a step change rather than a single spike.

The correlation id is the thread that ties a user-visible symptom to the project scaffolding decision and to the component log entry for the same operation. Without it, an investigation starts from a timestamp and a guess.

Watch the file renderer failure rate separately from the overall error rate. A jump in file renderer failures usually means a template that hardcodes a path and generates files outside the target directory, not a bug in variable prompt, and the two have different owners and different fixes.

Chart the share of project scaffolding operations that shed, queue, or succeed. A change in the mix points at the cause faster than an aggregate latency can, because each class has a different root.

Troubleshooting

A template that hardcodes a path and generates files outside the target directory. Reproduce with a minimal fixture, then add a guard test so the project scaffolding system rejects the condition loudly.

Performance regresses under project scaffolding load. Profile with the sampling analyzer, locate the hot path in variable prompt, and add a targeted benchmark to lock the regression.

Keep the project scaffolding dashboards separate from dependency dashboards. When an operation fails, the correlation id lets you decide in seconds whether the fault is template loader, variable prompt, or file renderer, instead of guessing from a timeout.

Limitations and next steps

• Single-region; multi-region replication is out of scope.
• The template loader is in-process and not horizontally shared by default.
• No auth layer; assume deployment behind an edge that enforces identity.

The project scaffolding system is single-region by design; extending to multi-region means making template loader shareable across regions, which is a real change to the consistency model rather than a config flag. The file renderer is in-process, which is correct for one instance but must move to a shared store before scaling out.

There is no authentication or authorization layer; the system assumes it is deployed behind an edge that enforces identity. Adding one is straightforward but out of scope for this build, and doing it here would distract from the project scaffolding correctness that is the point.

The natural next step, once the project scaffolding pipeline is trusted in production, is to add a short-TTL cache for idempotent reads and a richer hook runner that exports histograms rather than counters. Both are additive and neither changes the contracts this build establishes.