[英]Mocha / Chai expect.to.throw not catching thrown errors
我在让 Chai 的expect.to.throw
为我的 node.js 应用程序进行测试时遇到问题。 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 和 catch 中,并对捕获的错误进行断言,它就可以工作。
是否expect.to.throw
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.
你必须传递一个函数来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.'));
你正在做的方式,要传递到expect
调用的结果model.get('z')
但是,为了测试是否东西被抛出,你必须通过一个函数来expect
,这expect
会调用本身。 上面使用的bind
方法创建了一个新函数,在调用时将调用model.get
并将this
设置为model
的值,并将第一个参数设置为'z'
。
可以在此处找到对bind
一个很好的解释。
正如这个答案所说,您也可以将您的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.');
如果你已经在使用 ES6/ES2015,那么你也可以使用箭头函数。 它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.');
这个问题有很多很多重复,包括没有提到 Chai 断言库的问题。 以下是收集到的基础知识:
断言必须调用函数,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错误:
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.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./)
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);
}
});
乍一看,这可能令人困惑。 就像骑自行车一样,一旦发出咔嗒声,它就会永远“咔哒”一声。
文档中的示例... ;)
this
上下文: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
另一种可能的实现,比 .bind() 解决方案更麻烦,但有助于说明 expect() 需要一个为所覆盖的函数提供this
上下文的函数,您可以使用call()
,例如,
expect(function() {model.get.call(model, 'z');}).to.throw('...');
我找到了一个很好的方法:
// 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
});
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