在 mongodb 和 nodejs 中承诺未决错误

Question

I have written node.js code for getting some number using mongodb database.this is my code for that

    MongoClient.connect('mongodb://localhost:27017/mongomart', function(err, db) {

    assert.equal(null, err);

    var numItems=db.collection('item').find({"category":category}).count();

    callback(numItems);
});

This mongodb query runs correct on mongo shell but it is giving error when using with node.js

Promise <Pending>

I don't know what is this "promise" ? Please help..

Answer 1

node.js code is asynchronous so that numItems won't contain count of items - it rather contains Promise that contains count of items when resolved. You defenetely have to master the basics of node.js and asynchronous programming. Try to modify your code like this

MongoClient.connect('mongodb://localhost:27017/mongomart', function(err, db) {
  assert.equal(null, err);
  db.collection('item').find({"category":category}).count()
    .then(function(numItems) {
      console.log(numItems); // Use this to debug
      callback(numItems);
    })
});

For native Promise check out documentation https://developer.mozilla.org/ru/docs/Web/JavaScript/Reference/Global_Objects/Promise

Also look at bluebird promises https://github.com/petkaantonov/bluebird

Answer 2

A promise is a substitute temporary value that is given while you wait on the real value. To get the real value do

numItems.then(function (value) { callback(value) });

Or better yet, return the promise from your function, and let they implement it using the Promises pattern, instead of the callback pattern.

Answer 3

Had the same problem. Don't know if it's still relevant to you, but this is what solved it for me:

var category = 'categoryToSearch';
var cursor = db.collection('item').find({'category':category});

cursor.count(function (err, num) {
    if(err) {
        return console.log(err);
    }
    return num;
});

Answer 4

I drove myself bananas trying to solve a similar problem, where the document.save() option just gave Promise{pending} no matter what I did. Here's what I did:

• Change (req,res) to async(req,res) .
• Change var post = doc.save() to var post = await doc.save() .

Finally, log in to MongoDB web, and change accessible IP addresses to 0.0.0.0 (all addresses). Not doing this can cause issues sometimes even when your IP is whitelisted.

Answer 5

try this:

MongoClient.connect('mongodb://localhost:27017/mongomart', async (err, db) => {

    assert.equal(null, err);

    var numItems= await db.collection('item').find({"category":category}).count();

    callback(numItems);
});

(adding the await and turn this function to async function )