CODEWITHSUNDEEP
javascriptnode.jsamazon-web-servicesamazon-dynamodbes6-promise
StMartin
StMartin

Reputation: 89

Promise.all() - Rejection Id 2 - Not receiving data

I am seeing unexpected behavior when calling Promise.all(). I can't seem to get the desired data. When the promise is called directly the data returns just fine, but calling Promise.all() throws following error:

2018-04-09T16:31:35.827Z    77ea5111-3c13-11e8-9800-a5b8cf1bf96f    (node:1) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): TypeError: Cannot read property 'streak' of undefined

This is the relevant code:

function getUserData(UserId) {
  const docClient = new AWS.DynamoDB.DocumentClient();
  const params = {
    TableName: "XXXXXXXXXX",
    Key: {
      "UserId": UserId,
    }
  };

  return docClient.get(params).promise();
}

function buildResponse (UserId, weekNumber, yesterday, today, lastLogin) {
    let currentStreak

    getUserData(UserId);

    getUserData(UserId).then((data) => {    
    currentStreak = data.Item.streak;
    console.log("Streak is " + currentStreak); //Successfully returns "Streak is 22"
     });


    Promise.all([getUserData(UserId)]).then((data) => { //Throws error shown below
    var test = data.Item.streak;
    console.log("Promise is " + test);
    });

}

Note: I would like to add multiple promises in the future but I am just starting with one for now.

Upvotes: 0

Views: 79

Answers (2)

sripberger
sripberger

Reputation: 1732

As @Hunter pointed out, Promise.all resolves with an array of all individual promise results. Arrays don't have an Item property, so data.Item is undefined, thus accessing data.Item.streak gives you the error you're seeing.

It doesn't look like you should be using Promise.all(), though. You're only actually waiting on one promise, and unnecessarily wrapping it in an array. Try just taking it out, along with the array brackets:

getUserData(UserId).then((data) => {
    var test = data.Item.streak;
    console.log("Promise is " + test);
});

Upvotes: 0

Hunter McMillen
Hunter McMillen

Reputation: 61512

Promise.all() returns an array, you need to destructure the response: 

Promise.all([getUserData(UserId)]).then(([data]) => { 
  var test = data.Item.streak;
  console.log("Promise is " + test);
});

or: 

Promise.all([getUserData(UserId)]).then((response) => { 
  var [data] = response;
  var test = data.Item.streak;
  console.log("Promise is " + test);
});

or extract the data by index: 

Promise.all([getUserData(UserId)]).then((response) => { 
  var data = response[0];
  var test = data.Item.streak;
  console.log("Promise is " + test);
});

additionally you should have .catch() blocks to ensure that errors don't bubble up.

Upvotes: 2

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