NodeJS Returning data from nested promise

Question

I am fairly new to promises and could so with any pointers on how to get this to work. None of the variations I have tried have managed to trigger the final success/error callbacks, or have managed to but haven't returned the data.

export function getData(start, end, tagSet){
    let data = new dataSet();
    // Initial setup to work out what data to fetch

    return database.Read(config).then(function (results) {
        // Process the results.......
        return data;
    }, function (errCode) {
        // Throw Error
    });
}

// Call the function, triggered by a seperate event
getData.then(function(data){
    //Success !!
    
},function(err){
    //Failure!!
    
});

I have also tried setting up a new promise

export function getData(start, end, tagSet){
    let data = new dataSet();
    // Initial setup to work out what data to fetch

    return new Promise(function(resolve, reject) {
        database.Read(config).then(function (results) {
            // Process the results.......
            // Update data
        }, function (errCode) {
            // Throw Error
        });
        return data;
    }
}

I don't want to make the getData function blocking by using await , so what is the correct way to return data from a promise inside a function?

• Edit: Typo
• Looks like I misunderstood how await works when inside a async instruction.

Answer 1

You don't return anything from promises, but instead resolve or reject a promise.

your 2nd example should look something like this

    export function getData(start, end, tagSet){
    let data = new dataSet();
    // Initial setup to work out what data to fetch

    return new Promise(function(resolve, reject) {
        database.Read(config).then(function (results) {
            // Process the results.......
            // Update data
        }, function (errCode) {
            // Throw Error
            reject(errCode) // <-- if error happens, you reject your promise with error
        });
        resolve(data) // <<-- if there is no error, you resolve your data
    }
 }

then you can use

getData.then(function(data){
   //promise is resolved
})
.catch(function(err){
   //promise was rejected
})

Answer 2

Looks like the solution I wanted was to set the function as async and use an await.

export async function getData(start, end, tagSet){
    let data = new dataSet();
    // Initial setup to work out what data to fetch

    await database.Read(config).then(function (results) {
        // Process the results.......
        // update data
    }, function (errCode) {
        // Throw Error
    });
    
    return data;
}

// Call the function, triggered by a seperate event
getData().then(function(data){
    // Success !!
    // data has been completed here
},function(err){
    // Failure!!
    
});

This results in the returned data being completed before being the getData().then() is triggered.