What is the best way to retrieve a string from a lambda expression?

Question

What is the best way to retrieve a string from a lambda expression?

I am trying to call the function myFunction() and retrieve the source._id value, but I'm encountering issues with the current code. Even though source._id is correctly filled, I am unsure of how to successfully return it in its entirety. Ideally, something like:

var newId = myFunction();

Both the query and save operations are utilizing mongoose promises.

        var myFunction = () => {
            var query = MyModel.findOne({ user: userId, name: name.name });

            query.exec((err, doc) => {
                if (err) {
                    reject(err);
                } else {
                    if (doc != null) {
                        var msg = "Error Msg here";
                        reject(new ValidationError(msg));
                    } else {
                        var source = new MyModel();
                        source.someUserProp  = userId;
                        source.save((err, doc) => {
                            if (err) {
                                throw(err)
                            }
                            else { 
                                return(source._id);
                            }
                        });
                    }
                }
            })
        };

javascript lambda typescript mongoose promise

Answer 1

Answer №1

Utilize promises efficiently when handling asynchronous operations:

const myCustomFunction = () => {
    const query = MyModel.findOne({ user: userId, name: name.name });

    return query.exec().then(doc => {
        if (doc !== null) {
            const errorMessage = "Custom error message here";
            throw new ValidationError(errorMessage);
        }

        const newData = new MyModel();
        newData.someUserProp  = userId;

        return newData.save().then(() => newData._id);
    });
};

myCustomFunction()
    .then(id => console.log(id))
    .catch(error => console.error(error));

Answer 2

Utilize promises efficiently when handling asynchronous operations:

const myCustomFunction = () => {
    const query = MyModel.findOne({ user: userId, name: name.name });

    return query.exec().then(doc => {
        if (doc !== null) {
            const errorMessage = "Custom error message here";
            throw new ValidationError(errorMessage);
        }

        const newData = new MyModel();
        newData.someUserProp  = userId;

        return newData.save().then(() => newData._id);
    });
};

myCustomFunction()
    .then(id => console.log(id))
    .catch(error => console.error(error));

Answer 3

Answer №2

query.exec() and source.save() are asynchronous functions, meaning that when you try to return the source.id value, it actually goes back to these async functions rather than returning directly to your function. As far as I know, there is no straightforward way to pass a value from an async function up to your main function. However, here are a couple of approaches you can consider.

One option is to try returning the async functions themselves, which might provide you with the desired functionality.


var myFunction = () => {
    var query = MyModel.findOne({ user: userId, name: name.name });

    **return** query.exec((err, doc) => {
        if (err) {
            reject(err);
        } else {
            if (doc != null) {
                var msg = "Error message";
                reject(new ValidationError(msg));
            } else {
                var source = new MyModel();
                source.someUserProp  = userId;
                **return** source.save((err, doc) => {
                    if (err) {
                        throw err;
                    }
                    else { 
                        return (source._id);
                    }
                });
            }
        }
    })
};

If that doesn't work for you, another approach is to provide your function with a callback parameter so that you can access the value after the function has finished executing.


var myFunction = (callback) => {
    var query = MyModel.findOne({ user: userId, name: name.name });

    query.exec((err, doc) => {
        if (err) {
            reject(err);
        } else {
            if (doc != null) {
                var msg = "Error message";
                reject(new ValidationError(msg));
            } else {
                var source = new MyModel();
                source.someUserProp  = userId;
                source.save((err, doc) => {
                    if (err) {
                        throw err;
                    }
                    else { 
                        callback(source._id);
                    }
                });
            }
        }
    })
};

To call this function, you would do:


myFunction((id) => { //You can access 'id' inside this block });

Answer 4

query.exec() and source.save() are asynchronous functions, meaning that when you try to return the source.id value, it actually goes back to these async functions rather than returning directly to your function. As far as I know, there is no straightforward way to pass a value from an async function up to your main function. However, here are a couple of approaches you can consider.

One option is to try returning the async functions themselves, which might provide you with the desired functionality.


var myFunction = () => {
    var query = MyModel.findOne({ user: userId, name: name.name });

    **return** query.exec((err, doc) => {
        if (err) {
            reject(err);
        } else {
            if (doc != null) {
                var msg = "Error message";
                reject(new ValidationError(msg));
            } else {
                var source = new MyModel();
                source.someUserProp  = userId;
                **return** source.save((err, doc) => {
                    if (err) {
                        throw err;
                    }
                    else { 
                        return (source._id);
                    }
                });
            }
        }
    })
};

If that doesn't work for you, another approach is to provide your function with a callback parameter so that you can access the value after the function has finished executing.


var myFunction = (callback) => {
    var query = MyModel.findOne({ user: userId, name: name.name });

    query.exec((err, doc) => {
        if (err) {
            reject(err);
        } else {
            if (doc != null) {
                var msg = "Error message";
                reject(new ValidationError(msg));
            } else {
                var source = new MyModel();
                source.someUserProp  = userId;
                source.save((err, doc) => {
                    if (err) {
                        throw err;
                    }
                    else { 
                        callback(source._id);
                    }
                });
            }
        }
    })
};

To call this function, you would do:


myFunction((id) => { //You can access 'id' inside this block });

What is the best way to retrieve a string from a lambda expression?

Answer №1

Answer №2

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