Understanding the asynchronous behavior of Mongoose's findById method

Question

Understanding the asynchronous behavior of Mongoose's findById method

I want to discuss the code I am currently working on:

const checkForRecord = async (id) => {

    let model = mongoose.model('User');

    let query = {}
    query.dummy = false; <== This particular field is causing an error intentionally, as it does not exist in my User model.
    let result = await model.findById(id, query);

    console.log('This part of the code is being reached !!!');
    console.log(result);
}

However, I encounter the following error:

(node:6680) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): CastError: Cast to ObjectId failed for value ...

In addition, my console.log statements do not seem to execute.

Why is this error not being caught in the result variable, considering that my function is asynchronous?

I have tried both approaches:

let result = await model.findById(id, query);

and

let result = await model.findById(id, query).exec();

Yet, the outcome remains the same.

javascript asynchronous mongoose async-await

Answer 1

Answer №1

My console.logs are not getting executed at all.

This is the expected behavior. The function in question is asynchronous and it waits for a promise to be returned. In this scenario, rejections are treated as exceptions which cause the termination of the checkForRecord function.

Why isn't that error being assigned to result, considering my operation is async?

The reason is that the error is not a resolution value (which is what await provides); instead, it is a rejection or exception. Understanding the de-sugared version of checkForRecord by replacing async and await with their underlying promise operations may provide clarity:

// checkForRecord with async/await desugared to their underyling Promise operations
const checkForRecord = (id) => {

    let model = mongoose.model('User');

    let query = {};
    query.dummy = false; // <== To trigger an error, this field is not utilized
    return model.findById(id, query).then(value => {
        let result = value;
        console.log('Code reached here !!!');
        console.log(result);
    });
};

In the above code snippet, the console.log statements won't run because they're within a resolution handler. Rejections do not go to a resolution handler but to a rejection handler.

To clarify, there is no requirement to modify checkForRecord. The provided example demonstrates how async/await translates at runtime.

Your current implementation of checkForRecord is correct (except for missing => and absence of comments on the query.dummy line). You should utilize it as follows inside an async function:

try {
    checkForRecord(someId);
} catch (e) {
    // Handle the error here
}

Alternatively, if not within an async function, you can handle errors as shown below:

checkForRecord(someId).catch(e => {
    // Handle the error here
});

Answer 2

My console.logs are not getting executed at all.

This is the expected behavior. The function in question is asynchronous and it waits for a promise to be returned. In this scenario, rejections are treated as exceptions which cause the termination of the checkForRecord function.

Why isn't that error being assigned to result, considering my operation is async?

The reason is that the error is not a resolution value (which is what await provides); instead, it is a rejection or exception. Understanding the de-sugared version of checkForRecord by replacing async and await with their underlying promise operations may provide clarity:

// checkForRecord with async/await desugared to their underyling Promise operations
const checkForRecord = (id) => {

    let model = mongoose.model('User');

    let query = {};
    query.dummy = false; // <== To trigger an error, this field is not utilized
    return model.findById(id, query).then(value => {
        let result = value;
        console.log('Code reached here !!!');
        console.log(result);
    });
};

In the above code snippet, the console.log statements won't run because they're within a resolution handler. Rejections do not go to a resolution handler but to a rejection handler.

To clarify, there is no requirement to modify checkForRecord. The provided example demonstrates how async/await translates at runtime.

Your current implementation of checkForRecord is correct (except for missing => and absence of comments on the query.dummy line). You should utilize it as follows inside an async function:

try {
    checkForRecord(someId);
} catch (e) {
    // Handle the error here
}

Alternatively, if not within an async function, you can handle errors as shown below:

checkForRecord(someId).catch(e => {
    // Handle the error here
});

Understanding the asynchronous behavior of Mongoose's findById method

Answer №1

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