Apps Script 邊界測試：運費級距改了，怎麼知道沒改壞

/**
 * @OnlyCurrentDoc
 */

// 訂單金額 ≤500 運費 100；501 到 1000 運費 60；超過 1000 免運
const FEE_TABLE = [
  { max: 500, fee: 100 },
  { max: 1000, fee: 60 },
  { max: Infinity, fee: 0 },
];

// 回傳運費；金額無法判斷（空白、負數、不是數字）時回傳 null，不當成免運
function shippingFee(amount) {
  const isText = typeof amount === "string" && amount.trim() !== "";
  const n = typeof amount === "number" ? amount : isText ? Number(amount) : NaN;
  if (!Number.isFinite(n) || n < 0) {
    return null;
  }
  return FEE_TABLE.find((t) => n <= t.max).fee;
}

// 先定答案：每一筆都是「輸入」與「預期」，邊界的兩側都要有
const TEST_CASES = [
  { amount: 0, expected: 100 },
  { amount: 1, expected: 100 },
  { amount: 500, expected: 100 },
  { amount: 500.5, expected: 60 },
  { amount: 501, expected: 60 },
  { amount: 1000, expected: 60 },
  { amount: 1000.01, expected: 0 },
  { amount: 1001, expected: 0 },
  { amount: "500", expected: 100 },
  { amount: "", expected: null },
  { amount: -1, expected: null },
  { amount: "abc", expected: null },
];

function runShippingTests() {
  const book = SpreadsheetApp.getActiveSpreadsheet();
  const sheet = book.getSheetByName("測試結果") || book.insertSheet("測試結果");
  sheet.clearContents();
  const show = (value) => (typeof value === "string" ? '"' + value + '"' : String(value));
  let passed = 0;
  const rows = TEST_CASES.map((testCase) => {
    const actual = shippingFee(testCase.amount);
    const ok = actual === testCase.expected;
    if (ok) {
      passed++;
    }
    return [show(testCase.amount), String(testCase.expected), String(actual), ok ? "PASS" : "FAIL"];
  });
  sheet.getRange(1, 1, rows.length + 1, 4).setValues([["輸入", "預期", "實際", "結果"]].concat(rows));
  Logger.log("通過 " + passed + "/" + rows.length);
}