Question

Overview

So I have pulled out a document from my database. Inside is a nested collection of objects. Each of the objects inside this nested collection has an ‘_id’ attribute. I want to find one on these objects specifically by its ‘_id’ using Javascript.

Example

http://jsfiddle.net/WilsonPage/tNngT/

Alternative Example

http://jsfiddle.net/WilsonPage/tNngT/3/

Questions

1. Is my example the best way of achieving this?
2. Will this block in Node.js?

Answer 1

Yes, if you only know a specific value which is contained by one of your objects (which are held in an Array) you need to loop over the whole structure and compare those values.

As you also did right, break the iteration when you found one (return in your example).
So my answer to your first question would be yes, in terms of performance this is the right and best way.

What I don’t get is the “Async” example. You just moved the code and changed the structure. Your code is still “blocking” since you’re using a normal for-loop to search. If that array would be huge, it would block your node-app for the amount of time the loop needs to finish.

To really make it asynchronously, you would need to get rid of any loop. You would need to loop over the structure with a runway-timer.

var findById = function(collection, _id, cb){
    var coll = collection.slice( 0 ); // create a clone

    (function _loop( data ) {
        if( data._id === _id ) {
            cb.apply( null, [ data ] );
        }
        else if( coll.length ) {
            setTimeout( _loop.bind( null, coll.shift() ), 25 );
        }
    }( coll.shift() ));
};

And then use it like

findById( myCollection, 102, function( data ) {
    console.log('MATCH -> ', data);
});

This technique (which is a simplified example), we are creating an self-invoking anonymous function and we pass in the first array item (using .shift()). We do our comparison and if we found the item we are looking for, execute a callback function the caller needs to provide. If we don’t have a match but the array still contains elements (check for the .length), we create a timeout of 25ms using setTimeout and call our _loop function again, this time with the next array item, because .shift() gets and removes the first entry. We repeat that until either no items are left or we found the element. Since setTimeout gives other tasks in the JS main thread (on a browser, the UI thread) the chance to do things, we don’t block and screw up the whole show.

As I said, this can get optimized. For instance, we can use a do-while loop within the _loop() method and also use Date.now() to do things until we go over the 50ms mark for instance. If we need longer than that, create a timeout the same way and repeat the above described operation again (so its like, do as much operation as possible within 50ms).