测试含内部函数的JavaScript函数时遇返回值断言错误求助

Let's break down why your test is failing and how to fix it:

The Root Problem #

Your withinMiles function doesn't return a boolean value directly—it returns another function (the arrow function (customer) => calculateMiles(...) <= max). When you run expect(withinMiles(250, 500)).toBe(true), you're comparing that returned function to the boolean true, which is why you get the Expected: true, Received: [Function anonymous] error.

On top of that, there's a parameter mismatch: your withinMiles expects the first argument to be an object with a distance property (since it uses miles.distance), but you're passing a raw number (250) instead. That would cause calculateMiles to receive undefined as the second argument, leading to unexpected results even after fixing the test structure.

Corrected Test Case #

Here's how to adjust your test to work properly:

test("Customer is within 500 miles", () => {
  // 1. Create a test customer with a distance value
  const testCustomer = { distance: 700 };
  // 2. Get the range-checking function from withinMiles
  //    Pass an object with distance as the first argument, not a raw number
  const isWithinRange = withinMiles({ distance: 250 }, 500);
  // 3. Call the returned function with your test customer and assert the result
  expect(isWithinRange(testCustomer)).toBe(true);
});

How It Works #

1. Create a test customer: We define a customer object with a distance property, which matches what the returned arrow function expects.
2. Retrieve the checker function: withinMiles({ distance: 250 }, 500) returns the function that will evaluate whether a customer's distance falls within the allowed range.
3. Execute the checker and assert: We call isWithinRange(testCustomer) to get the boolean result, then assert that it equals true.

Bonus: Add a Negative Test #

To make your test suite more robust, you can add a case where the customer is outside the allowed range:

test("Customer is outside 500 miles", () => {
  const testCustomer = { distance: 800 };
  const isWithinRange = withinMiles({ distance: 250 }, 500);
  expect(isWithinRange(testCustomer)).toBe(false);
});

内容的提问来源于stack exchange，提问作者user6248190