Iterating over data from mongoose in node - node.js

Question

I have this fiddle that illustrates what I want to do and it works as intended:
https://jsfiddle.net/mbLwsnak/
All it does is loop through an array of objects, and adds a key value pair.
I have this same code in my node app, and it doesn't work. The new pair is not added to the objects, even though the code is practically the same, so I figure it must be the context.
Schema.find()
.then(function(currencydata){
var data = currencydata;
data.forEach(function(obj){
obj.thing = "Thing";
})
console.log(data);
mdata.currencydata = data;
})
.then(function(result){
res.render('index', mdata);
})
So the only difference I can think of that matters is that this is all inside an asynchronous function. I even tried putting the log inside a setTimeout in case it was something to do with being async, but no change.
Edit:
currencydata is an array of objects returned by the mongoose schema. It looks like this:
[{
country: 'Canada',
currency: 'CAD',
cost: 5.98,
exchangerate: 1.33,
value: -10.9
}, {
country: 'United States',
currency: 'USD',
cost: 5.06,
exchangerate: 1,
value: 0
}, {
country: 'India',
currency: 'INR',
cost: 170,
exchangerate: 68.33,
value: -50.8
}]

Answer 1

Use lean() to convert a mongoose document into a plain javascript
object.
Then inside the first then() you need to return a promise so that the
next then() will get the returned value.
Inside the promise object, the resolve and reject functions, when called, resolve or reject the promise, respectively. Promise Reference
Schema.find()
.lean()
.then((currencydata, err) => {
return new Promise((resolve, reject) => {
currencydata.forEach(data => {
data.thing = 'Thing';
});
resolve(currencydata);
});
})
.then((newdata, err) => {
res.render('index', newdata);
});