摩卡/柴的Expect.to.throw不能捕获抛出的错误

我在让Chai的expect.to.thrownode.js应用程序进行测试时遇到问题。测试会因抛出的错误而不断失败，但是如果我将测试用例包装在try和catch中并断言所捕获的错误，它将起作用。

难道expect.to.throw不喜欢的工作，我认为它应该还是什么？

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.

您必须将一个函数传递给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.'));

执行此操作的方式将传递给call expect的结果model.get('z')。但是要测试是否抛出了某些东西，您必须将一个函数传递给expect，该函数expect会自行调用。bind上面使用的方法创建了一个新函数，当调用该函数时，将model.get使用this设置为的值model和设置为的第一个参数调用'z'。

bind可以在此处找到有关的很好的解释。

就像这个答案所说，您还可以将代码包装在一个匿名函数中，如下所示：

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

而且，如果您已经在使用ES6 / ES2015，则还可以使用箭头功能。它与使用常规匿名函数基本相同，但更短。

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

这个问题有很多重复项，包括未提及Chai断言库的问题。这里是一起收集的基础知识：

断言必须调用该函数，而不是立即求值。

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的错误：

节点

  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./)

应该

  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./)

柴期望

  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“逃脱”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);
  }
});

首先，这看起来会令人困惑。就像骑自行车一样，一旦点击，它就会永远“点击”。

doc中的示例...;）

因为您依赖this上下文：

• .throw调用函数时丢失的内容
• 它没有办法知道这应该是什么

您必须使用以下选项之一：

• 将方法或函数调用包装在另一个函数中

• 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

还有一种可能的实现，比.bind（）解决方案更麻烦，但是可以帮助您指出Expect（）需要一个this为覆盖的函数提供上下文的函数，您可以使用call()例如

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

我发现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
});