node bluebird promise chaining

Question

I'm new to promise. I am trying to use promise to send queries to mysql db. After some queries I will use the result from the query, do some calculations and then use the output as some parameters of next query. Looks like the following:

firstQuery(). then(secondQuery). then(thirdQuery). then(fourthQuery). ...

Say, in the fourthQuery, I need to use results coming from firstQuery and secondQuery, and will have some additional calculations. How should I do that?

I know I can get the result from the previous promise by passing a parameter to the function:

then(thirdQuery). then(cal("I can only get output from thirdQuery here")). then(fourthQuery("pass output from cal"))

In this case, I don't any advantages of Promise over callbacks, because I can always write a function to simplify the repeated callbacks.

Answer 1

If you can rewrite firstQuery, secondQuery etc you can do something like this

function firstQuery(allResult = {}) {
    return doTheQuery()
    .then(result => {
        allResult.first = result;
        return result;
    });
}
function secondQuery(allResult = {}) {
    return doTheQuery()
    .then(result => {
        allResult.second = result;
        return result;
    });
}
function thirdQuery(allResult = {}) {
    return doTheQuery()
    .then(result => {
        allResult.third = result;
        return result;
    });
}
function fourthQuery(allResult = {}) {
    return doTheQuery(allRessult.first, allResult.second)
    .then(result => {
        allResult.fourth = result;
        return result;
    });
}

then you can write use

firstQuery()
.then(secondQuery)
.then(thirdQuery)
.then(fourthQuery)
.then ...

the final result will be an object with the values of all the queries in {first, second, third, fourth} properties

Of course, if you want just the fourth query result

firstQuery()
.then(secondQuery)
.then(thirdQuery)
.then(fourthQuery)
.then(result => result.fourth)
.then ...