How to handle async operations inside a while loop with Node.js?

Question

For a Node API I must generate a random alphanumeric key, this should be unique and SHORT (I can't use neither uuid or Mongo ObjectID).

I thought this logic:

Generate key,
Query MongoDB for key existance
If key exists, repeat the process,
If key doesn't exist, assign it and respond to client.

I tried then:

do {
  key = randomKey(8);
  newGroup.key = key;
  Group.findOne({numberId: key}).then(function (foundGroup) {
    console.log("cb");

    if (! foundGroup) {
      console.log("not found")
      notUnique = false;
    }

  }).catch(function (err) {
    return response.send(500);
  });

} while (notUnique);

But only I got is an infinite loop, notUnique is never switching to true. Just in case, this was tested against an empy database.

How could I achieve it?

Can you generate a lot of keys to test? If so, you can query them in bulk with the $in operator. You'll probably find less objects than the keys you generated and then you simply gotta check whether a key exists for which the corresponding object does not. — laggingreflex
– laggingreflex, Commented Jun 26, 2015 at 23:32

hassansin · Accepted Answer · 2015-06-26 23:13:59Z

2

You can do this easily with async module:

var async = require('async')

async.forever(
    function(next) {
        key = randomKey(8);

        Group.findOne({numberId: key}).then(function (foundGroup) {
          console.log("cb");

          if (! foundGroup) {
            console.log("not found")
            notUnique = false;
            next(new Error(), key) // break forever loop with empty error message and the key
          }
          else 
            return next() //continue loop

        }).catch(function (err) {
          next(err); //break loop
          return response.send(500);
        });
    },
    function(err, key) {
        if(key){
           newGroup.key = key;
        }

    }
);

answered Jun 26, 2015 at 23:13

hassansin

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1 Comment

Matt Browne Over a year ago

This is probably the most practical approach. FYI, there's a proposal for a future version of Javascript (ECMAScript 7) called async/await that would allow you to write this more concisely, and you can use it today thanks to Babel: babeljs.io/docs/advanced/transformers/other/async-to-generator

laggingreflex · Accepted Answer · 2015-06-27 02:06:42Z

Since you're already using promises, I'd do something like this: Create a function that recursively returns a promise, creating a promise chain, and eventually throws an error when a condition is/isn't met. You then just need to catch that final error.

edit: updated to return the key

function find(){
    var key = randomKey(8);
    return Group.findOne({numberId: key })
    .then(function(foundGroup) {
        if(foundGroup) return find(); // recursion
        else throw key;
    }).catch(function(err){ // in case Group.findOne throws
        throw key; 
    });
}
find().catch(function(key){
    return response.send(key);
});

The find function here will keep calling itself recursively as long as it keeps finding a valid object. And since it returns a promise they will all be chained automatically.

If and eventually when it doesn't find the object, or Group.findOne throws an error, the key is thrown.

find().catch will catch that eventual error, which will be the key.

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