assertcheck-feature

name	assertcheck-feature
description	Guide a developer building a new TypeScript feature to design it with Negative Space Programming from the start. Trigger when the user says "I'm building a new feature", "create a new function/class/service", "scaffold a new module", or starts writing a new function signature without assertions.

assertcheck-feature

References — load before starting

• references/contract-questions.md
• references/guard-templates.md

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LAWS

LAW 1 — Interview before any code. Never produce a contract table, guard block, or implementation until Q1, Q2, and Q3 are answered. Skip questions the user already answered — ask only what's missing.

LAW 2 — Contract table before guard block. Phase 1 (contract table) must be complete before Phase 2 (guard block) is written. Every row in the contract table must have a non-empty "Assert with" column. Unsure? Load assertcheck-selector.

LAW 3 — Guard block before logic. Always. The // ── guards ── block is the first thing in every function body. No logic before the last assertion. No exceptions.

LAW 4 — No ?., ??, or if (!x) return in the logic block. The negative space is declared in the guard block. The logic block runs with proven invariants. If you're tempted to add a nil check in the logic block, it belongs in the guard block.

LAW 5 — Every guard must have msg. Add note when the fix is non-obvious. msg describes the invariant ("orderId must be a non-empty string"). note gives the actionable fix ("check that the caller passes a valid order id").

LAW 6 — Deliver in order: contract table → implementation → rejected states summary. Never skip the rejected states summary. One sentence per assertion.

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Triggers

• "I'm creating a new function that…"
• "New service / class / module for…"
• "How do I write this feature with NSP?"
• User shares a function signature or empty function body

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Interview — ask before any output

1. What is the entry point? (function / HTTP handler / class method / constructor)
2. What are the inputs and where do they come from?
   (user input / HTTP body / database / env var / another service)
3. For each input: what makes it invalid? (null, empty, wrong type, out of range…)
4. Does the feature depend on a prior state? (e.g. order must be pending)
5. Does it call an external service / DB? What must be true about the response?
6. What must the output guarantee? (non-null id, positive total, specific status…)

See references/contract-questions.md for the extended checklist by feature type.

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Protocol — 3 phases

Phase 1 — Contract table

Fill from interview answers. Every row must have "Assert with".

| Boundary          | Invalid state                     | Assert with                     |
|:------------------|:----------------------------------|:--------------------------------|
| param `X`         | null, empty string, wrong type    | assert.string + assert.notEmpty |
| prior state       | entity not in expected status     | assert.equal(entity.status, …)  |
| external response | missing required field            | assert.notNil(res.field, {…})   |
| output            | computed value out of valid range | assert.greater(total, 0)        |

Phase 2 — Guard block first

import { assert, check } from "assertcheck"
// docs: https://thonymg.github.io/assertcheck/

function featureName(param1: Type, param2: Type): ReturnType {
  // ── guards — declare what must never enter ───────────────────
  // one assertion per contract table row, in order

  // ── logic — runs with proven invariants ──────────────────────
  // no ?., no ??, no if (!x) return
}

Use references/guard-templates.md for common patterns. Load assertcheck-selector to pick the right assertion.

Phase 3 — Logic block

No defensive checks. The negative space is already declared above.

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Canonical examples

❌ BEFORE — TypeScript + optional chaining + silent returns:

function filterProductsByCategory(
  products?: Product[] | null,
  category?: ProductCategories
) {
  return products?.filter(product => product?.category === category)
  // null products → returns undefined silently. Caller has no idea.
}

✅ AFTER — guard block declares the negative space:

import { assert, check } from "assertcheck"

function filterProductsByCategory(
  products: Product[],
  category: ProductCategories
): Product[] {
  // ── guards ────────────────────────────────────────────────────
  check(products)
    .notEmpty({
      msg:  "products must be a non-empty array",
      note: "check that the upstream service returned results before calling this",
    })
    .allInstanceOf(Object, "all products must be valid objects")

  assert.string(category, "category must be a string")

  // ── logic ─────────────────────────────────────────────────────
  return products.filter(p => p.category === category)
}

✅ Service method with state guard:

function createOrder(customerId: string, items: CartItem[]): Order {
  // ── guards ────────────────────────────────────────────────────
  assert.string(customerId,   "customerId must be a non-empty string")
  assert.notEmpty(customerId, "customerId must not be empty")

  check(items)
    .notEmpty("cart must have at least one item before creating an order")
    .all(i => i.quantity > 0, {
      msg:  "all items must have a positive quantity",
      note: "remove items with quantity ≤ 0 before calling createOrder()",
    })

  // ── logic ─────────────────────────────────────────────────────
  const total = items.reduce((sum, i) => sum + i.price * i.quantity, 0)
  return { id: crypto.randomUUID(), customerId, items, total, status: "pending" }
}

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SELF-CHECK — run before delivering

• Interview: Q1, Q2, Q3 answered before any code was produced
• Contract table: every row has a non-empty "Assert with" column
• Guard block: appears before the first line of logic in every function
• Logic block: contains no ?., ??, or if (!x) return
• Every assertion has msg with domain context (not "value is null")
• Output delivered in order: contract table → implementation → rejected states summary
• Rejected states summary: one sentence per assertion

If any item fails → fix before delivering.