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Run npx skillmds@latest add jagreehal/fn-args-deps in your terminal (requires Node.js), paste this page's agent-chat prompt into Claude, Cursor, or any MCP-connected agent, or download the SKILL.md file and copy it into your agent's skills directory.

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Is Fn Args Deps free to use?

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Who published Fn Args Deps?

jagreehal (@jagreehal) published this skill. Their other Agent Skills are listed on their SkillMD profile.

Fn Args Deps by jagreehal — Agent Skill

Name: Fn Args Deps
Rating: 4.5 (4 reviews)
Author: jagreehal

Fn Args Deps

Enforces the fn(args, deps) pattern: business logic as functions with explicit dependency injection instead of service classes. Use this skill when writing or refactoring TypeScript business logic, replacing classes, structuring deps, or setting up a composition root. Do not use when/for React component structure (use react-development) or Result/error modeling alone (use result-types).

jagreehal Updated Sep 10, 2026

File contents

Functions Over Classes: fn(args, deps)

Critical rules

Business logic is fn(args, deps) — never service classes. args are per-call; deps are long-lived collaborators.
Declare a per-function deps type. No god AllServiceDeps objects.
Wire deps once at a composition root. Handlers call factories, not constructors mid-request.
Inject only network/disk/clock collaborators. Import pure utilities directly.
Use import type for infra interfaces; instances arrive via deps.
Before writing examples or grouping choices, read references/examples.md and references/patterns.md.

Workflow

Identify business logic that should be a function (or extract it from a class).
Split per-call data into args and collaborators into a named XDeps type.
Before choosing individual vs grouped injection, read references/patterns.md.
Implement fn(args, deps). Keep classes only as thin framework wrappers or stateful resources.
Add a factory / composition-root wire-up so call sites never construct deps.
Test with mock<XDeps>() — no class instantiation. See references/examples.md.

Resources

references/examples.md — WRONG vs CORRECT signatures, composition root, NestJS wrapper, tests. Read when implementing or reviewing.
references/patterns.md — grouping, migration, when classes are OK, enforcement, rationalizations. Read before structuring modules.

Validation

Every business function is fn(args, deps) with a per-function deps type
No unused deps fields; no god deps bag
Deps wired once at composition root
Infra via import type only; instances via deps
Pure utilities imported, not injected
Remaining classes are wrappers, stateful resources, or builders
Tests use mock<XDeps>() without instantiating a class

Constraints

Framework-mandated classes, connection pools, caches, and fluent builders may stay as thin wrappers over pure functions.
Related: result-types, validation-boundary, testing-strategy, writing-tests, observability, strict-typescript, pattern-enforcement.

---name: fn-args-depsdescription: Enforces the fn(args, deps) pattern: business logic as functions with explicit dependency injection instead of service classes. Use this skill when writing or refactoring TypeScript business logic, replacing classes, structuring deps, or setting up a composition root. Do not use when/for React component structure (use react-development) or Result/error modeling alone (use result-types).---# Functions Over Classes: fn(args, deps)## Critical rules- Business logic is `fn(args, deps)` — never service classes. `args` are per-call; `deps` are long-lived collaborators.- Declare a per-function deps type. No god `AllServiceDeps` objects.- Wire deps once at a composition root. Handlers call factories, not constructors mid-request.- Inject only network/disk/clock collaborators. Import pure utilities directly.- Use `import type` for infra interfaces; instances arrive via `deps`.- Before writing examples or grouping choices, read [references/examples.md](references/examples.md) and [references/patterns.md](references/patterns.md).## Workflow1. Identify business logic that should be a function (or extract it from a class).2. Split per-call data into `args` and collaborators into a named `XDeps` type.3. Before choosing individual vs grouped injection, read [references/patterns.md](references/patterns.md).4. Implement `fn(args, deps)`. Keep classes only as thin framework wrappers or stateful resources.5. Add a factory / composition-root wire-up so call sites never construct deps.6. Test with `mock<XDeps>()` — no class instantiation. See [references/examples.md](references/examples.md).## Resources- [references/examples.md](references/examples.md) — WRONG vs CORRECT signatures, composition root, NestJS wrapper, tests. Read when implementing or reviewing.- [references/patterns.md](references/patterns.md) — grouping, migration, when classes are OK, enforcement, rationalizations. Read before structuring modules.## Validation- [ ] Every business function is `fn(args, deps)` with a per-function deps type- [ ] No unused `deps` fields; no god deps bag- [ ] Deps wired once at composition root- [ ] Infra via `import type` only; instances via `deps`- [ ] Pure utilities imported, not injected- [ ] Remaining classes are wrappers, stateful resources, or builders- [ ] Tests use `mock<XDeps>()` without instantiating a class## Constraints- Framework-mandated classes, connection pools, caches, and fluent builders may stay as thin wrappers over pure functions.- Related: `result-types`, `validation-boundary`, `testing-strategy`, `writing-tests`, `observability`, `strict-typescript`, `pattern-enforcement`.

Frequently asked questions