javascript - 如何测试 Promise.all() 上的 catch block ？

Question

得到这个代码:

//Waiting until all records are inserted to DB
Promise.all(promises).then(function(err){   
    context.succeed({ valid: true, succeedRecords:succeedRecords, failedRecords:failedRecords });
    //Log
    console.log("Lambda function is finished - Successfully added records: " + succeedRecords.length + ",  Failed records: " + 
        failedRecords.length + "  -  Total processed records: " + event.Records.length);   
})
.catch(function (err) {
    if (err) {
        var msg = "Got error from Promise.all(), Error: " + err;
        console.log(msg);
        context.fail({ valid: false, message: msg });
    }
});

我需要知道如何使用 mocha 测试 catch block ？

Answer 1

假设您只想检查 Promise.all 是否失败，代码会简单得多:

//Waiting until all records are inserted to DB
return Promise.all(promises).then(function(err){   
  context.succeed({ valid: true, succeedRecords:succeedRecords, failedRecords:failedRecords });
  //Log
  console.log("Lambda function is finished - Successfully added records: " + succeedRecords.length + ",  Failed records: " + 
    failedRecords.length + "  -  Total processed records: " + event.Records.length);   
});

在这种情况下，

mocha 会将 catch 附加到返回的 Promise 中，如果调用则失败。

在某些情况下，您希望处理错误场景，执行一些检查并仅在特定场景下失败:

//Waiting until all records are inserted to DB
return Promise.all(promises).then(function(err){   
  context.succeed({ valid: true, succeedRecords:succeedRecords, failedRecords:failedRecords });
  //Log
  console.log("Lambda function is finished - Successfully added records: " + succeedRecords.length + ",  Failed records: " + 
    failedRecords.length + "  -  Total processed records: " + event.Records.length);   
})
.catch(function (err) {
  var msg = "Got error from Promise.all(), Error: " + err;
  console.log(msg);
  // ... do some stuff here
  context.fail({ valid: false, message: msg });

  // add an expectation here to check if the error message is correct
  // If the error message is "this error" the test passes, otherwise it will fail
  expect(err).to.be.eql('this error');
});

详细

我将退一步并尝试详细解释它是如何工作的。假设您有一个 Promise 被拒绝的测试:

// your code
it('rejects', () => {

  return Promise.reject();

});

此测试将失败，因为 mocha 将执行以下操作(实际代码为 here ):

// this is pseudo code for the mocha insides
var result = fn.call(ctx);
if (result && result.then) {

  result
    .then(function successCase() {
      // all fine here
      done();
    })
    .catch(function errorCase(reason) {
      done(reason);
    });
  }

因此，您返回的 Promise 将与 .then 和 .catch 链接起来，并且将传递两个回调:一个用于成功(使测试通过)，另一个用于一个表示错误(使测试失败)。

在上述拒绝案例中，发生的情况如下:

  // mocha code (I've just replaced the function call with its result)
  var result = Promise.reject();
  if(result && result.then){
    result.then(function successCase() {
      // this is not called!
      done();
    })
    .catch(function errorCase(reason) {
      // it will land here!
      done(reason);
    } );
  }

在OP问题的代码中，Promise可能会出错，并且该错误将被捕获:

Promise.reject()
  .catch( function( err ){
    var msg = "Got error from Promise.all(), Error: " + err;
    console.log(msg);
    context.fail({ valid: false, message: msg });
  }); <-- what is the output here?

以上是您的 Promise.all 被拒绝并且您在测试中捕获它的情况。 catch 的结果是...成功!

// your code
Promise.reject()
  .catch( function( err ){
    var msg = "Got error from Promise.all(), Error: " + err;
    console.log(msg);
    context.fail({ valid: false, message: msg });
  })

  // mocha code
  .then(function successCase() {
    // this is called! (and make the test pass)
    done();
  })
  .catch(function errorCase(reason) {
    // it will NOT call this
    done(reason);
  });

现在，假设您希望 Promise 拒绝，捕获错误，然后测试某些内容(也许是错误消息？其他选项？等等...)。鉴于上面的代码，你如何做到这一点？很简单，你以某种方式抛出错误。一种方法可能是显式引发错误，或者您可以使用断言(失败的断言将引发错误)。

// your code
Promise.reject()
  .catch( function( err ){
    var msg = "Got error from Promise.all(), Error: " + err;
    console.log(msg);
    context.fail({ valid: false, message: msg });

    // It will throw!
    expect(true).to.be(false);

    // or you can go vanilla
    throw Error("I don't like Errors!");
  })

  // mocha code
  .then(function successCase() {
    // this is NOT called!
    done();
  })
  .catch(function errorCase(reason) {
    // it will call this! (and make the test fail)
    done(reason);
  });