Promise.promise的所有数组

Question

I have this code:

 db.group.findOne({_id: id}).then((groupFound) => {
     var membersArray = groupFound.members;

     Promise.all(membersArray.map((member) => {
         return db 
            .doneTodo
            .find({'victor._id': member._id})
            .then((userVictories) => {
                return {
                    email: member.email,
                    victories: userVictories.length
                }
            });
    })).then(function (result) {
        console.log(result);
    });

The db calls are all MongoDB/Mongoose.

The inside of the array returned from the map function is all promises.

The last call:

.then((userVictories) => {
    return {
        email: member.email,
        victories: userVictories.length
    }
})

returns a promise.

So essentially, after this call is made, what I have is:

[promise1, promise2, promise3, ... , promise x]

How are these promises concluded to change the array value to the value of promise1.then( (result)) ?

Answer 1

How are these promises concluded to change the array value to the value of promise1.then( (result))?

There's nothing special here. The promises within the array will act like any promise does - they will resolve to some value after a period of time, result in an error after some period of time, or if they were set up poorly, never resolve or produce an error.

The purpose of Promise.all is simply to wait for all of the values in the array to resolve or for one to throw an error. If they all resolve, the promise returned from Promise.all will resolve to an array of all the results. If one throws an error, it will produce that error.

Promise.all isn't doing anything to the promises, just watching them. We could create our own simple version of Promise.all like this:

 function myAll(arr) { var resolvedCount = 0; var resultArray = []; return new Promise(function (resolve, reject) { if (arr.length === 0) { resolve([]); } arr.forEach(function (el, i) { // wrap el in a promise in case it's not already one Promise.resolve(el) .then( function (result) { // add result to the result array and increment the count resultArray[i] = result; resolvedCount += 1; if (resolvedCount === arr.length) { // all promises have resolved, // so this promise can resolve itself resolve(resultArray); } }, function (err) { // a promise threw an error. reject reject(err); } ); // end of .then() }); // end of .forEach() }); // end of new Promise() } // test out myAll() function log(value) { console.log(value); } function logError(value) { log('Error caught:' + value); } myAll([]) .then(log, logError); myAll([1, Promise.resolve('apple'), 5, Promise.resolve(7)]) .then(log, logError); myAll([2, 3, Promise.reject('no!')]) .then(log, logError);