Get all values from a specific key within an array - JS

Question

I am trying to access all values from a specific key (_id) within an array. My array:

"users" : [
    { "_id" : "LqTE6we2TYaA3v23K" },  
    { "_id" : "knfoWfpn5Y9niSgae" },
    { "_id" : "NkHWuyRCpxCvCHJcA" },  
    { "_id" : "YQF6BaCA9Xc8aaYTY" },  
    { "_id" : "mceWvJgGfpH3XB4mh" },  
    { "_id" : "zAWoF3BiLpAAv4vmP" },       
    { "_id" : "c4fLw7TfkGu9jdbFT" } 
]

And I want this:

[ 'LqTE6we2TYaA3v23K', 'knfoWfpn5Y9niSgae', 'NkHWuyRCpxCvCHJcA', …]

Is it possible to achieve this without looping over the array? Any help would be greatly appreciated.

Do want a JavaScript method or a MongoDB method. With your current tags you are going to mostly get pure JavaScript responses so I hope that is what you want. — Neil Lunn
– Neil Lunn, Commented Apr 18, 2014 at 10:21

cocco · Accepted Answer · 2014-04-18 10:37:24Z

1

map was added to the ECMA-262 standard in the 5th edition, so not long time ago.

Also the peformance is not one of the best.

http://jsperf.com/map-vs-native-for-loop/2

So if you want performance & compatibility:

var array=[],l=users.length;
while(l--){array[l]=users[l]._id}
// now array contains your id's

is faster and the code is not much longer.

while loop is the fastest on most browsers
direct setting the array index is also faster than push if you already have an index

demo

http://jsfiddle.net/vmfju/

Naturally the advantage of map is that it's a inline function based on new technologies and easier to read. So @Hatsjoem is also a good choice.

edited Apr 18, 2014 at 10:37

answered Apr 18, 2014 at 10:19

cocco

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Comments

Neil Lunn · Accepted Answer · 2014-04-18 11:01:41Z

0

As stated really not sure what you want as commented based on the tagging of your question. All the JavaScript methods of course involve "looping" in some form or another regardless of the method used.

Possibly actually meaning getting the data from MongoDB? Well you can, and I'm just presuming you are actually using Meteor based on the _id values so I'm using that syntax for it's specific call to the aggregate method:

db.collection("collection").aggregate([
    { "$unwind": "$users" },
    { "$group": {
        "_id": "$_id",
        "users": { "$push": "$users._id" }
    }}
])

So that will just pull the specific _id field out of the documents in the array and $push them back into an array that just contains those values. So you can then send the response without needing to "post-process" the removal of that _id key from the result.

So if you are indeed using Meteor, then you need to access the raw collection and not just the "Meteor collection" object which does not give access to the aggregate method. For anything close to a native driver, then whatever your normal syntax is.

answered Apr 18, 2014 at 11:01

Neil Lunn

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5 Comments

user3475602 Over a year ago

1. Thanks for the help! Yes, I am using Meteor. My problem: I want to subscribe only to a specific entries in a collection. It is called: notifications. In my user collection, I store a reference to the main notification with its _id. I was thinking to get the array of user's notification id's in the iron-router configure (before) method and then return Meteor.subscribe('notifications', notificationsIdArray) in the waitOn function. However, this is not 100% reactive. I don't know why, but only if I add 2 notifications the changes will be published to the client (or with site refresh).

user3475602 Over a year ago

2. Since I need this for subscribing and publishing data, I need to put this in my /lib/router.js and therefore I can't access the raw collection with db.

Neil Lunn Over a year ago

@user3475602 Yes you can. This is the "inner" part of the code that you are going to "publish" so yes to can, just like any arbitrary code. So if you really do want this as a "published" function then it is absolutely possible. Better than the client side iterations anyhow.

user3475602 Over a year ago

Do I need to define db? This might be a silly question, but I am new to Meteor and MongoDB and if I try your code snippet, I will get the error: can't find variable db.

Neil Lunn Over a year ago

@user3475602 Sorry if not completely concise but yes, you need a handle to the underlying connection and then everything is actually the "node native driver" objects and methods as opposed to the wrappers provided by Meteor. But it can of course be done.

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