JavaScript：具有Promise的递归函数，resolve返回“ undefined”

Question

I have a recursive function which is returning promise, there is some conditions which on fulfillment the resolve will fire. But the resolve always returns undefined.

Here is the code:

var children = [];
var temp = [];
function getRelationsOfRep(repId) {
    var index;
    return new Promise(function (resolve, reject) {
        return RepRelation.findOne({_id: repId}).then(function (repData) {
            if (<condition>) {
                temp = temp.concat(repData.children);
                temp.forEach(function (childId) {
                    return getRelationsOfRep(childId);
                });
            } else {
                // else
            }
            if (<another condition>) {
                return resolve(children);
            }
        }).catch(function (error) {
            return reject(error);
        })
    })
}

function updateRepData(id){
    children = [];
    temp = [];
    getRelationsOfRep(id).then(function (branchesData) {
        console.log('branchesData:: ', branchesData); // it prints undefined
    })
}

I'm trying to get all children of a Representative and the children of its children (if any) and return them all.

What is it that I'm doing wrong?

Any help would be much appreciated.

Thanks in advance.

Answer 1

You are making things slightly complicated. I think what you want to achieve is to use promise to return a result from a recursive function. Here's a cleaner version

A cleaner solution -

getRelationsOfRep(repId) {
    return new Promise( function(resolve,reject) {
          recursiveFunctionCall(repId);
          function recursiveFunctionCall(repId) {
             //your recursive function logic
             recursiveFunctionCall(childId);
             if(...) //edge case condition
               resolve('your-answer');
             else 
               reject('your-error')
          }
    });

This way, you'll just be using one promise and returning when your recursive function resolves.

Answer 2

Use Promise.all() to capture all the promises in Loop.

Instead of:

temp.forEach(function (childId) {
    return getRelationsOfRep(childId);
});

Try:

var promisesArray = [];
temp.forEach(function (childId) {
    promisesArray.push(getRelationsOfRep(childId));
});
return Promise.all(promisesArray);

Using something like this, you will be able to get nested response for the recursive calls.

PS Not tested, modify according to the logic you want.