Question

得到这段代码：

//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块？

Answer 1

假设您只想检查Promise.all是否失败，代码会更简单：

//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会向返回的promise附加catch，如果被调用则会失败。

在某些情况下，您希望处理错误情况，执行一些检查并仅在特定情况下失败：

//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

我会退后一步，试着详细解释它的工作原理。假设你有一个拒绝承诺的测试：

// 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，您返回的承诺将与.then和.catch链接，并且将传递两个回调：一个用于成功（使测试通过），一个用于错误（使测试失败））。

在上述拒绝案件中，会发生以下情况：

  // 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，说你想要拒绝承诺，捕获错误然后测试一些东西（可能是错误信息？其他一些选项？等等......）。鉴于上面的代码你怎么做？很容易，你以某种方式抛出错误。一种方法可能是显式抛出错误，或者您可以使用断言（失败的断言会引发错误）。

// 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);
  });

如何在Promise.all（）上测试catch块？

1 个答案:

详细