js-06-business-days

# Business days between (date-fns)

Implement an ES module **`solution.js`** that uses **date-fns** (already installed — import it):

```js
import { parseISO, eachDayOfInterval, isWeekend } from "date-fns";

export function businessDaysBetween(startISO, endISO) { /* ... */ }
```

Given two ISO date strings (`"YYYY-MM-DD"`), count the number of **business days**
(weekdays Monday–Friday) in the range **(start, end]** — that is, **exclusive of the
start date and inclusive of the end date**.

Rules:
- Saturday and Sunday are not business days.
- The start date itself is never counted (even if it is a weekday).
- The end date is counted if it is a weekday.
- If `end <= start`, return `0`.

Use date-fns helpers such as `parseISO`, `eachDayOfInterval`, and `isWeekend`.

Examples:
```js
businessDaysBetween("2024-01-01", "2024-01-05") // => 4
// Mon..Fri: start Mon excluded; Tue, Wed, Thu, Fri counted

businessDaysBetween("2024-01-05", "2024-01-08") // => 1
// Fri..Mon: Fri excluded, Sat/Sun weekend, Mon counted

businessDaysBetween("2024-01-01", "2024-01-01") // => 0
// same day

businessDaysBetween("2024-01-01", "2024-01-08") // => 5
// one week: Mon excluded; Tue–Fri (4) + next Mon (1)
```

import { test } from "node:test";
import { strict as assert } from "node:assert";
import { businessDaysBetween } from "./solution.js";

test("Mon..Fri excludes start, counts Tue-Fri", () => {
  assert.equal(businessDaysBetween("2024-01-01", "2024-01-05"), 4);
});

test("Fri..Mon skips the weekend, counts Mon", () => {
  assert.equal(businessDaysBetween("2024-01-05", "2024-01-08"), 1);
});

test("same day -> 0", () => {
  assert.equal(businessDaysBetween("2024-01-01", "2024-01-01"), 0);
});

test("end before start -> 0", () => {
  assert.equal(businessDaysBetween("2024-01-10", "2024-01-01"), 0);
});

test("one full week (Mon..Mon) -> 5", () => {
  assert.equal(businessDaysBetween("2024-01-01", "2024-01-08"), 5);
});

test("range entirely within a weekend -> 0", () => {
  // Sat 2024-01-06 .. Sun 2024-01-07
  assert.equal(businessDaysBetween("2024-01-06", "2024-01-07"), 0);
});

test("start on a weekend still excludes start, counts weekdays after", () => {
  // Sat 2024-01-06 .. Fri 2024-01-12: Mon-Fri (5) counted
  assert.equal(businessDaysBetween("2024-01-06", "2024-01-12"), 5);
});

test("two weeks spanning two weekends", () => {
  // Mon 2024-01-01 .. Mon 2024-01-15: exclude first Mon.
  // Tue-Fri wk1 (4) + Mon-Fri wk2 (5) + final Mon (1) = 10
  assert.equal(businessDaysBetween("2024-01-01", "2024-01-15"), 10);
});

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.

```js
import { parseISO, eachDayOfInterval, isWeekend } from "date-fns";

export function businessDaysBetween(startISO, endISO) {
  const startDate = parseISO(startISO);
  const endDate = parseISO(endISO);

  if (endISO <= startISO) {
    return 0;
  }

  const interval = { start: startDate, end: endDate };
  const days = eachDayOfInterval(interval);

  let count = 0;

  for (const day of days) {
    if (!isWeekend(day)) {
      count++;
    }
  }

  return count;
}
```

[
  {
    "total": 8,
    "answer": "```js\nimport { parseISO, eachDayOfInterval, isWeekend } from \"date-fns\";\n\nexport function businessDaysBetween(startISO, endISO) {\n  const startDate = parseISO(startISO);\n  const endDate = parseISO(endISO);\n\n  if (endISO <= startISO) {\n    return 0;\n  }\n\n  const interval = { start: startDate, end: endDate };\n  const days = eachDayOfInterval(interval);\n\n  let count = 0;\n\n  for (const day of days) {\n    if (!isWeekend(day)) {\n      count++;\n    }\n  }\n\n  return count;\n}\n```",
    "passed": 4,
    "test_error": "e:internal/test_runner/harness:385:3)\n  ...\n# Subtest: same day -> 0\nok 3 - same day -> 0\n  ---\n  duration_ms: 0.093321\n  type: 'test'\n  ...\n# Subtest: end before start -> 0\nok 4 - end before start -> 0\n  ---\n  duration_ms: 0.057081\n  type: 'test'\n  ...\n# Subtest: one full week (Mon..Mon) -> 5\nnot ok 5 - one full week (Mon..Mon) -> 5\n  ---\n  duration_ms: 0.115501\n  type: 'test'\n  location: '/tmp/llmlab-js-06-business-days-udrl_nsi/solution.test.js:21:1'\n  failureType: 'testCodeFailure'\n  error: |-\n    Expected values to be strictly equal:\n    \n    6 !== 5\n    \n  code: 'ERR_ASSERTION'\n  name: 'AssertionError'\n  expected: 5\n  actual: 6\n  operator: 'strictEqual'\n  stack: |-\n    TestContext.<anonymous> (file:///tmp/llmlab-js-06-business-days-udrl_nsi/solution.test.js:22:10)\n    Test.runInAsyncScope (node:async_hooks:227:14)\n    Test.run (node:internal/test_runner/test:1306:25)\n    Test.processPendingSubtests (node:internal/test_runner/test:897:18)\n    Test.postRun (node:internal/test_runner/test:1447:19)\n    Test.run (node:internal/test_runner/test:1372:12)\n    async Test.processPendingSubtests (node:internal/test_runner/test:897:7)\n  ...\n# Subtest: range entirely within a weekend -> 0\nok 6 - range entirely within a weekend -> 0\n  ---\n  duration_ms: 0.066801\n  type: 'test'\n  ...\n# Subtest: start on a weekend still excludes start, counts weekdays after\nok 7 - start on a weekend still excludes start, counts weekdays after\n  ---\n  duration_ms: 0.052251\n  type: 'test'\n  ...\n# Subtest: two weeks spanning two weekends\nnot ok 8 - two weeks spanning two weekends\n  ---\n  duration_ms: 0.07609\n  type: 'test'\n  location: '/tmp/llmlab-js-06-business-days-udrl_nsi/solution.test.js:35:1'\n  failureType: 'testCodeFailure'\n  error: |-\n    Expected values to be strictly equal:\n    \n    11 !== 10\n    \n  code: 'ERR_ASSERTION'\n  name: 'AssertionError'\n  expected: 10\n  actual: 11\n  operator: 'strictEqual'\n  stack: |-\n    TestContext.<anonymous> (file:///tmp/llmlab-js-06-business-days-udrl_nsi/solution.test.js:38:10)\n    Test.runInAsyncScope (node:async_hooks:227:14)\n    Test.run (node:internal/test_runner/test:1306:25)\n    Test.processPendingSubtests (node:internal/test_runner/test:897:18)\n    Test.postRun (node:internal/test_runner/test:1447:19)\n    Test.run (node:internal/test_runner/test:1372:12)\n    async Test.processPendingSubtests (node:internal/test_runner/test:897:7)\n  ...\n1..8\n# tests 8\n# suites 0\n# pass 4\n# fail 4\n# cancelled 0\n# skipped 0\n# todo 0\n# duration_ms 492.971485"
  },
  {
    "total": 8,
    "answer": "```js\nimport { parseISO, eachDayOfInterval, isWeekend } from \"date-fns\";\n\nexport function businessDaysBetween(startISO, endISO) {\n  const startDate = parseISO(startISO);\n  const endDate = parseISO(endISO);\n\n  if (endISO <= startISO) {\n    return 0;\n  }\n\n  const interval = { start: startDate, end: endDate };\n  const days = eachDayOfInterval(interval);\n\n  let count = 0;\n\n  for (const day of days) {\n    if (!isWeekend(day)) {\n      count++;\n    }\n  }\n\n  return count;\n}\n```",
    "passed": 4,
    "test_error": "node:internal/test_runner/harness:385:3)\n  ...\n# Subtest: same day -> 0\nok 3 - same day -> 0\n  ---\n  duration_ms: 0.09369\n  type: 'test'\n  ...\n# Subtest: end before start -> 0\nok 4 - end before start -> 0\n  ---\n  duration_ms: 0.05574\n  type: 'test'\n  ...\n# Subtest: one full week (Mon..Mon) -> 5\nnot ok 5 - one full week (Mon..Mon) -> 5\n  ---\n  duration_ms: 0.112671\n  type: 'test'\n  location: '/tmp/llmlab-js-06-business-days-mrme9d0j/solution.test.js:21:1'\n  failureType: 'testCodeFailure'\n  error: |-\n    Expected values to be strictly equal:\n    \n    6 !== 5\n    \n  code: 'ERR_ASSERTION'\n  name: 'AssertionError'\n  expected: 5\n  actual: 6\n  operator: 'strictEqual'\n  stack: |-\n    TestContext.<anonymous> (file:///tmp/llmlab-js-06-business-days-mrme9d0j/solution.test.js:22:10)\n    Test.runInAsyncScope (node:async_hooks:227:14)\n    Test.run (node:internal/test_runner/test:1306:25)\n    Test.processPendingSubtests (node:internal/test_runner/test:897:18)\n    Test.postRun (node:internal/test_runner/test:1447:19)\n    Test.run (node:internal/test_runner/test:1372:12)\n    async Test.processPendingSubtests (node:internal/test_runner/test:897:7)\n  ...\n# Subtest: range entirely within a weekend -> 0\nok 6 - range entirely within a weekend -> 0\n  ---\n  duration_ms: 0.0652\n  type: 'test'\n  ...\n# Subtest: start on a weekend still excludes start, counts weekdays after\nok 7 - start on a weekend still excludes start, counts weekdays after\n  ---\n  duration_ms: 0.053181\n  type: 'test'\n  ...\n# Subtest: two weeks spanning two weekends\nnot ok 8 - two weeks spanning two weekends\n  ---\n  duration_ms: 0.071821\n  type: 'test'\n  location: '/tmp/llmlab-js-06-business-days-mrme9d0j/solution.test.js:35:1'\n  failureType: 'testCodeFailure'\n  error: |-\n    Expected values to be strictly equal:\n    \n    11 !== 10\n    \n  code: 'ERR_ASSERTION'\n  name: 'AssertionError'\n  expected: 10\n  actual: 11\n  operator: 'strictEqual'\n  stack: |-\n    TestContext.<anonymous> (file:///tmp/llmlab-js-06-business-days-mrme9d0j/solution.test.js:38:10)\n    Test.runInAsyncScope (node:async_hooks:227:14)\n    Test.run (node:internal/test_runner/test:1306:25)\n    Test.processPendingSubtests (node:internal/test_runner/test:897:18)\n    Test.postRun (node:internal/test_runner/test:1447:19)\n    Test.run (node:internal/test_runner/test:1372:12)\n    async Test.processPendingSubtests (node:internal/test_runner/test:897:7)\n  ...\n1..8\n# tests 8\n# suites 0\n# pass 4\n# fail 4\n# cancelled 0\n# skipped 0\n# todo 0\n# duration_ms 489.902245"
  },
  {
    "total": 8,
    "answer": "```js\nimport { parseISO, eachDayOfInterval, isWeekend } from \"date-fns\";\n\nexport function businessDaysBetween(startISO, endISO) {\n  const startDate = parseISO(startISO);\n  const endDate = parseISO(endISO);\n\n  if (endISO <= startISO) {\n    return 0;\n  }\n\n  const interval = { start: startDate, end: endDate };\n  const days = eachDayOfInterval(interval);\n\n  let count = 0;\n\n  for (const day of days) {\n    if (!isWeekend(day)) {\n      count++;\n    }\n  }\n\n  return count;\n}\n```",
    "passed": 4,
    "test_error": ""
  }
]