承诺：然后再解决

Question

Very first thing I've ever done in Node.js, I'm writing an AWS Lambda function, and I want to check whether a custom attribute on a User has a value before doing anything else. Since I'm told Promises are the way to handle asynchronous methods synchronously, I wrote the function:

var AWS = require('aws-sdk');
var s3 = new AWS.S3();
var cogId = new AWS.CognitoIdentityServiceProvider();

exports.handler = function (event, context) {

    if (event != null)
    {
        var identityId = context.identity.cognitoIdentityId;

        if (event.userId != null)
        {
            var userId = event.userId;
            PromiseConfirmIdNotSet(userId)
                .then(SetId(userId, identityId))
                .catch();
        }
    }

    context.done(null, 'Hello World');  // SUCCESS with message
};

function PromiseConfirmIdNotSet(userId)
{
    console.log('Entering function');
    return new Promise(function (resolve, reject) {
        console.log('Entering Promise');
        cogId.adminGetUser({
                UserPoolId: myUserPool,
                UserId: userId
            },
            function (err, data) {
                console.log('err = ' + JSON.stringify(err));
                console.log('data = ' + JSON.stringify(err));
                if (data != null && data.UserAttributes.Name == null) {
                    console.log('Calling resolve');
                    resolve();
                } else {
                    console.log('Calling reject');
                    reject();
                }
            });
    });
    console.log('Exiting Promise');
}

function SetId(userId, identityId)
{
    cogId.updateUserAttributes();
}

But when I run it, the console log shows "Entering function", then "Entering Promise", then the execution goes to SetId without ever having called the callback specified in adminGetUser.

If I let the debugger continue after the main flow is done, eventually I do get the logs from the callback function, so it does eventually run.

Why is the Promise skipping to the then without the resolve ever getting called?

Answer 1

.then accepts a function as an argument. When you do

PromiseConfirmIdNotSet(userId)
  .then(SetId(userId, identityId))
  .catch();

PromiseConfirmIdNotSet is called, and synchronously, SetId is called, while the interpreter tries to construct a Promise chain from the function passed to .then. (But SetId doesn't return a function) Then, after that, PromiseConfirmIdNotSet's asynchronous code runs, and the Promise resolves - which isn't in the order you want.

Change it so that SetId is only called after the promise returned by PromiseConfirmIdNotSet resolves:

PromiseConfirmIdNotSet(userId)
  .then(() => SetId(userId, identityId))
  .catch();

The problem is similar to why

addEventListener('click', fn());

doesn't work - you'd change it to , fn); or , () => fn());.

If you additionally want context.done to occur only after a successful SetId, then put the context.done call inside the .then:

PromiseConfirmIdNotSet(userId)
  .then(() => {
    SetId(userId, identityId);
    context.done(null, 'Hello World');  // SUCCESS with message
  });