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) {
  // Parse the ISO date strings to Date objects
  const startDate = parseISO(startISO);
  const endDate = parseISO(endISO);
  
  // If end date is on or before start date, return 0
  if (endDate <= startDate) {
    return 0;
  }
  
  // Get all days in the interval (start, end] - exclusive start, inclusive end
  // Since eachDayOfInterval includes both endpoints, we need to adjust:
  // We want from (startDate + 1 day) to endDate
  const nextDay = new Date(startDate);
  nextDay.setDate(nextDay.getDate() + 1);
  
  // If nextDay is after endDate, there are no days in the interval
  if (nextDay > endDate) {
    return 0;
  }
  
  // Get all days from nextDay to endDate (inclusive)
  const days = eachDayOfInterval({ start: nextDay, end: endDate });
  
  // Count business days (weekdays only)
  let count = 0;
  for (const day of days) {
    if (!isWeekend(day)) {
      count++;
    }
  }
  
  return count;
}
```