调用函数是调用promise函数并将结果返回给原始函数的JavaScript

Question

I have two functions (One function within the controller and oen function in a service). I want to call the service function from the controller function. After calling teh service function this function calls a database function (using Sequelize).

Controller:

exports.getMilestoneById = async (req, res, next) => {
    const milestoneId = req.params.milestoneId;
    try{
    milestoneService.getMilestoneById(milestoneId)
    .then(mielstone => {
        console.log("Milestone in tehn: " + mielstone)
    });
    console.log("Milestone: " + milestone);
    res.status(200).json({message: 'Milestone fetched', milestone: milestone});
    } catch (error){
        if (!error.statusCode){
            error.statusCode = 500;
        }
    }
}

Service:

exports.getMilestoneById = async (milestoneId) => {

Milestone.findByPk(milestoneId)
.then(milestone => {
    console.log("In Service: " + milestone)
    return milestone.get();
})
.catch(err => {
    console.log("Error" + err);
});

}

The problem: I don't get a milestone back.

Answer 1

Ok, let's try to put some order here.

Promises Flow

Your code doesn't wait for the Promise response.

milestoneService.getMilestoneById(milestoneId) // <- this returns a promise
// all stuff below here doesn't wait for the promise to complete and can't see its result
console.log("Milestone: " + milestone);
res.status(200).json({message: 'Milestone fetched', milestone: milestone});

Instead, writing something like:

milestoneService.getMilestoneById(milestoneId)
    .then(mielstone => {
        console.log("Milestone in tehn: " + mielstone);
        // I moved all the stuff that needs to wait in here!!
        console.log("Milestone: " + milestone);
        res.status(200).json({message: 'Milestone fetched', milestone: milestone});
    });

This would work as expected.

Async / Await

You added that async before the functions, I'd figure you could put that to good use! async / await helps you clean the promises flow by letting the wrapping function wait for their results, so writing something like:

    const milestone = await milestoneService.getMilestoneById(milestoneId)
    console.log("Milestone: " + milestone);
    res.status(200).json({message: 'Milestone fetched', milestone: milestone});

Would also work as expected.

Returns

... or not! You missed pretty much all returns in your code. As a rule of thumbs try to make every function always return something, especially if it's promises! So like:

exports.getMilestoneById = async (milestoneId) => {
 const milestone = await Milestone.findByPk(milestoneId);
 return milestone.get();
 /* I don't suggest handling the error here, because it doesn't bubble well! 
 /* If there's an error what would this function return?
}

Then

exports.getMilestoneById = async (req, res, next) => {
    const milestoneId = req.params.milestoneId;
    try {
       const milestone = await milestoneService.getMilestoneById(milestoneId)
       console.log("Milestone: " + milestone);
        // just return this
       return res.status(200).json({ message: 'Milestone fetched', milestone });
    } catch (error){
        if (!error.statusCode){
            error.statusCode = 500;
        }
        // call next with the error!
        return next(error);
    }
}

Answer 2

When you are returning the result in this callback:

.then(milestone => {
    console.log("In Service: " + milestone)
    return milestone.get();
})

You are in fact setting the return value for the callback function, not the return value for your service function milestone service.getMilestoneById() . Because your function is an async function, you should use Javascript's async-await syntax, which will only get the value once the promise is resolved.

exports.getMilestoneById = async (milestoneId) => {
    const milestone = await Milestone.findByPk(milestoneId)
    .catch(err => {
        console.log("Error" + err);
    });

    console.log("In Service: " + milestone)
    return milestone
}

This should be the same for your controller function:

exports.getMilestoneById = async (req, res, next) => {
    const milestoneId = req.params.milestoneId;
    try{
    const milestone = await milestoneService.getMilestoneById(milestoneId)
    console.log("Milestone: " + milestone);
    res.status(200).json({message: 'Milestone fetched', milestone: 
    milestone});
    } catch (error){
        if (!error.statusCode){
            error.statusCode = 500;
        }
    }
}