Mocha / Chai expect.to.throw 没有捕捉到抛出的错误

Question

我无法让 Chai 的 expect.to.throw 在我的 node.js 应用程序的测试中工作。测试在抛出的错误上一直失败，但是如果我将测试用例包装在 try 中并捕获并断言捕获的错误，它就可以工作。

expect.to.throw 是否不像我认为的那样工作？

it('should throw an error if you try to get an undefined property', function (done) {
  var params = { a: 'test', b: 'test', c: 'test' };
  var model = new TestModel(MOCK_REQUEST, params);

  // neither of these work
  expect(model.get('z')).to.throw('Property does not exist in model schema.');
  expect(model.get('z')).to.throw(new Error('Property does not exist in model schema.'));

  // this works
  try { 
    model.get('z'); 
  }
  catch(err) {
    expect(err).to.eql(new Error('Property does not exist in model schema.'));
  }

  done();
});

失败：

19 passing (25ms)
  1 failing

  1) Model Base should throw an error if you try to get an undefined property:
     Error: Property does not exist in model schema.

Answer 1

您必须将函数传递给expect。像这样：

expect(model.get.bind(model, 'z')).to.throw('Property does not exist in model schema.');
expect(model.get.bind(model, 'z')).to.throw(new Error('Property does not exist in model schema.'));

按照您的操作方式，您将调用model.get('z') 的结果传递给expect。但是要测试是否有东西被抛出，你必须将一个函数传递给expect，expect 会调用它自己。上面使用的bind 方法创建了一个新函数，调用该函数时将调用model.get，其中this 设置为model 的值，第一个参数设置为'z'。

bind 的一个很好的解释可以找到here。

Answer 2

作为this answer says，您也可以像这样将代码包装在匿名函数中：

expect(function(){
    model.get('z');
}).to.throw('Property does not exist in model schema.');

Answer 3

如果你已经在使用 ES6/ES2015，那么你也可以使用箭头函数。与使用普通匿名函数基本相同，但更短。

expect(() => model.get('z')).to.throw('Property does not exist in model schema.');

Answer 4

这个问题有很多很多重复项，包括未提及 Chai 断言库的问题。以下是汇总的基础知识：

断言必须调用函数，而不是立即计算。

assert.throws(x.y.z);      
   // FAIL.  x.y.z throws an exception, which immediately exits the
   // enclosing block, so assert.throw() not called.
assert.throws(()=>x.y.z);  
   // assert.throw() is called with a function, which only throws
   // when assert.throw executes the function.
assert.throws(function () { x.y.z });   
   // if you cannot use ES6 at work
function badReference() { x.y.z }; assert.throws(badReference);  
   // for the verbose
assert.throws(()=>model.get(z));  
   // the specific example given.
homegrownAssertThrows(model.get, z);
   //  a style common in Python, but not in JavaScript

您可以使用任何断言库检查特定错误：

Node

  assert.throws(() => x.y.z);
  assert.throws(() => x.y.z, ReferenceError);
  assert.throws(() => x.y.z, ReferenceError, /is not defined/);
  assert.throws(() => x.y.z, /is not defined/);
  assert.doesNotThrow(() => 42);
  assert.throws(() => x.y.z, Error);
  assert.throws(() => model.get.z, /Property does not exist in model schema./)

Should

  should.throws(() => x.y.z);
  should.throws(() => x.y.z, ReferenceError);
  should.throws(() => x.y.z, ReferenceError, /is not defined/);
  should.throws(() => x.y.z, /is not defined/);
  should.doesNotThrow(() => 42);
  should.throws(() => x.y.z, Error);
  should.throws(() => model.get.z, /Property does not exist in model schema./)

Chai Expect

  expect(() => x.y.z).to.throw();
  expect(() => x.y.z).to.throw(ReferenceError);
  expect(() => x.y.z).to.throw(ReferenceError, /is not defined/);
  expect(() => x.y.z).to.throw(/is not defined/);
  expect(() => 42).not.to.throw();
  expect(() => x.y.z).to.throw(Error);
  expect(() => model.get.z).to.throw(/Property does not exist in model schema./);

您必须处理“逃避”测试的异常

it('should handle escaped errors', function () {
  try {
    expect(() => x.y.z).not.to.throw(RangeError);
  } catch (err) {
    expect(err).to.be.a(ReferenceError);
  }
});

起初这看起来很混乱。就像骑自行车一样，它只会在咔哒一声后永远“咔哒”一声。

Answer 5

来自doc...的示例；）

因为你依赖this 上下文：

• 当函数被.throw调用时丢失
• 它无法知道这应该是什么

您必须使用以下选项之一：

• 包装在另一个函数中的方法或函数调用

• 绑定上下文

  // wrap the method or function call inside of another function
  expect(function () { cat.meow(); }).to.throw();  // Function expression
  expect(() => cat.meow()).to.throw();             // ES6 arrow function
  
  // bind the context
  expect(cat.meow.bind(cat)).to.throw();           // Bind
  

Answer 6

另一种可能的实现，比 .bind() 解决方案更麻烦，但有助于说明 expect() 需要一个为覆盖函数提供 this 上下文的函数，您可以使用 @ 987654322@，例如，

expect(function() {model.get.call(model, 'z');}).to.throw('...');

Answer 7

我找到了一个很好的解决方法：

// The test, BDD style
it ("unsupported site", () => {
    The.function(myFunc)
    .with.arguments({url:"https://www.ebay.com/"})
    .should.throw(/unsupported/);
});


// The function that does the magic: (lang:TypeScript)
export const The = {
    'function': (func:Function) => ({
        'with': ({
            'arguments': function (...args:any) {
                return () => func(...args);
            }
        })
    })
};

它比我的旧版本更具可读性：

it ("unsupported site", () => {
    const args = {url:"https://www.ebay.com/"}; //Arrange
    function check_unsupported_site() { myFunc(args) } //Act
    check_unsupported_site.should.throw(/unsupported/) //Assert
});