用 mocha 和 chai 测试“投掷”

Question

In my function I use:

  if (apples === undefined) {
    throw new Error('Error');
  }

In my test I am trying to confirm this is working by doing:

  it('throw an error if apples is undefined', function() {
    const input = undefined;
    const output = "Error";
    assert.throws(myFunc(input), output);
  });

However I get:

1 failing

1. myFunc should throw an error if apples is undefined: Error Error

Answer 1

That's because assert.throws 1st parameter takes the function definition , not the function result . It calls the passed function itself. In your example, you're calling the function therefore you're passing the function result.

Instead of this:

assert.throws(myFunc(), output)

Do it like this:

assert.throws(myFunc, output)

If you want to pass parameters to the function under test, you'll need to wrap it in another unnamed function or use Function.bind to bind some arguments to it.

Here's a working example:

const assert = require("assert")

function myFunc(apples) {
  if (!apples) {
    throw new Error("Apples must have a non-null value");
  }

  return apples + " are good";
}


describe("myFunc", () => {
  it("throws an error if apples is undefined", function() {
    const input = undefined;
    const output = new Error("Error");

    assert.throws(function() {
      myFunc(input);
    }, output);
  })
})

Note that you also need to test that it throws an instance of an Error instead of a String .