Promise.all not working at expected

Question

I expected the Promise.all code to return

{ constant: 'x1' }
{ constant: 'x2' }
{ constant: 'x3' }

but instead I have this:

{ constant: 'x1' }
{ constant: 'x12' }
{ constant: 'x123' }

Why would this happen? I can not seem to see where 'constant' is accumulating.

This is a simplified view - I actually updated/create a document in "testFN"

thanks

function testFN(blob, array, iter){
    var newBlob = blob;
    newBlob.constant = blob.constant+array.num
    console.log(newBlob);
    return products.findOneAndUpdate ....   
}

exports.updateProductId = function(req, res) {

    var blob = {"constant":"x"};
    var arr = { key1:{num: "1"}, key2:{num: "2"}, key3:{num: "3"}};
    var fnArr = [];

    Object.keys(arr).forEach(function(key, i) {
       fnArr.push(testFN(blob, arr[key], i));
    });

    return Promise.all(fnArr)
    .then(function(success){
        console.log(success);
    })
    .catch(function(error){
        console.log(error);
    })

})

Answer 1

Non-primitives are passed by reference - you can think of any variable that represents an object as a reference to a memory location . So, in testFN :

function testFN(blob, array, iter){
  var newBlob = blob;
  newBlob.constant = blob.constant+array.num

newBlob points to the same place in memory as the blob parameter, so when you change newBlob.constant , you change the original blob.constant as well (since they're the same object). You can fix it by assigning a true copy of blob to newBlob :

 function testFN(blob, array, iter){ var newBlob = JSON.parse(JSON.stringify(blob)); newBlob.constant = blob.constant + array.num; console.log(newBlob); return newBlob; } var blob = {"constant":"x"}; var arr = { key1:{num: "1"}, key2:{num: "2"}, key3:{num: "3"}}; var fnArr = []; Object.keys(arr).forEach(function(key, i) { fnArr.push(testFN(blob, arr[key], i)); }); Promise.all(fnArr) .then(function(success){ console.log(success); }) .catch(function(error){ console.log(error); }) 

This has nothing to do with Promise.all . None of this code is asynchronous anyway.

Answer 2

Promise.all can work with array of Promises change you function "testFN" to return Promise.

function testFN(blob, array, iter){
    return new Promise(function(resolve, reject) {
       var newBlob = blob;
       newBlob.constant = blob.constant+array.num;
       console.log(newBlob);
       resolve();
    });
}

But you might what to see if you need Promise.all in this case at all. Bcause you do not have any Async operation here