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How do I push mongodb data?

So I have developed a web app as a hobby on Handlebars, Express and Mongoose/MongoDB.
The app let's users create an account and then post advertisements for other users to see and respond to.
All the ads posted by users show up on the index page. So it is common view for all the users on this web app. I am relatively new to web development so to build such a simple app it took me a while but boy I learned a lot!
Now the issue I am facing is, when a user A posts an Ad while the user B is logged in and is currently on the index page (a page that lists all the ads posted) it won't show up for user B unless user B refreshes the page. Rightly so actually because only when the index page's route is hit it will query all the ads and refreshing is basically hitting the index route I get that. But I don't want it that way. I want it to show the new ad on user B's index and pretty much every user's index if there's new ad by any user.
So I did a little research/reading and I learned that I can do it by learning to work with triggers on mongodb and like create some kind of trigger that when a new ad is posted do something. I like the idea but failed to find resources to learn how to use such a thing.
The other option I was suggested was to use socket.io but that too I can't grasp how can I make an entire Ad document work as a socket. I am lost and implementing this feature of dynamically loading ads for all users will complete this hobby project of mine and will help me find a junior dev job in local community.
I request stackoverflow's community to guide me how do I go about doing this and what resources I can use to learn about it.

The socket.io seems to be the best solution for your case. What you will want to do with socket.io is every time a user posts an Ad you use socket.io to notify the rest of the users that there is an update.
If you don't want to send the entire document using the socket you can use the socket to notify the clients and on the client side every time you receive such a notice from the server you will either
a) Refresh the page(not suggested as it will make the user experience unpleasant) which is easier to implement
OR
b) You can use an Ajax request to get the new data from your server and update the fields on the fly(which makes for a better user experience).

Best Way You can come with using Short Polling concept from client side to ask for new data after 1 or 2 seconds (whatsoever count to need ) . Gmail for new inbox mails also uses sync method in a particular fashion . Just ask from server for new data
OR Second option to go through below
On Server Side
Serve index.html page to User A (which is logged in now).Some User B inserts data
Maintain a function or a cron job (checks the count of Total Ads ) Lets say after every 1 minute or so
If there is change in count from the previous total_count , update it and get new mlab documents and send it to function , Let's say push_new_ads which will be sync via socket.io to client
On Client Side
Sync your client_total_count with server_local_count push_new_ads using socket.io and If there is change in count , make a simple fetch api call to get data and appends it to previously fetched array
There is no such way to directly listen the changes in mongodb But you can trigger some changes from oplog using tailable cursors

Api call request limit of 1 call each hour

I use an API in Switzerland, which allows me to request the api one time every hour in production.
I don't need more than one request each week, since it's event data, but I don't know what i have to do that i can use this api for 200+ users each day.
Do I have to save the data somewhere like firebase or are there services for this? I'm very new in this field. Could you give me a hint?

Building on top of what Dr. cool said, you'll most likely want to use cron jobs: http://code.tutsplus.com/tutorials/scheduling-tasks-with-cron-jobs--net-8800
Also keep in mind, some API's do not allow you to store the data they provide on your own server. Make sure you read the terms of use before doing so from the API provider.

It's better to have a program on the server that can run once a week and load data from the API. This data should be saved in a database. Then when one of your users needs the data, it's ready to load from your database without hitting the API limit.
Yes, Firebase is a great option. Or you can use MySQL or other server-side databases.

Run PHP page every hour automatically-without using Cron jobs

How can i run or refresh PHP page every 10 Minutes or an hour. so that add new record from one database to another i doesn't matter with using javascript but without using cron jobs.

You can depend on the traffic to your website if it's frequent enough. Drupal has something called poor man's cron. Basically you store the date-time that the cron job was last run. Each time a user visits a page on your website, you fetch the date-time, compare it with the current date-time to see if you need to run the cron job (in your case, see if an hour has passed). If the required amount of time has passed, then run your cron job, and store a new date-time. If your required amount of time hasn't passed, then do nothing.
This approach has a few caveats.
You really should be using cron, and not doing this. There's a reason it's called poor man's cron. If you're using some hosting service by someone else, their technical support should be able to help you setup a cron job. If the service has cPanel you can do it from cPanel. https://documentation.cpanel.net/display/ALD/Cron+Jobs
You are completely dependent on website traffic to trigger your cron job. You need to decide if it's OKay for your cron job to not be run for a period of time where users are not visiting your website.
You're slowing every request to your website down by a fetch to whatever persistent store is holding the last date-time the cron job was run.

with javascript you can use the setTimeout() function but the user will need to keep the page opened.
i.e
setTimeout(function(){ your_process(); }, 360000); // every hour

Caching JavaScript API Calls

I'm querying the GitHub API from the client using JavaScript (on this page).
There are 14 API calls each time the page loads, which means I will end up hitting GitHub's API rate limit of 5000 calls per hour pretty fast.
Most caching strategies I've seen assume that you have access to a server, but in my case I'm running a purely static Middleman site.
So my question is this: how can I cache API requests from the client? Are there third-party apps that provide this service?
(Note that my use case is many different clients hitting the page (e.g. it has been linked from Hacker News), not a single client refreshing. So local caching wouldn't really help much. )

Agreed with Firebase or separate data store alternative from #David so you can create a persistent cache mechanism since you don't have access to the server where the application sits. It's basically another data store and you can update your logic in Middleman to either make a fresh call to the Github api or to pull from data saved in Firebase based on some checks you do when a person visits that Translation page. Check out the logic here

You can cache a single client's page by using local storage or a cookie. This way if the user refreshes, you can have logic to see if you want to query the API again. This would be fine if your user base was small.
This type of caching is typically done on the server since you are limiting yourself to ~357 users per hour at best.
To cache on the client side, store the data in local storage and log the time of the query. Then decide on an interval (let's say 5 minutes). Then prior to any refresh or page load, look a the users local storage and see if the query was within the last 5 minutes. If it was, read from the local storage. If not, then query the API again. This only applies to each user but by querying every 5 minutes, it would allow you to say ~30 users per hour.
http://diveintohtml5.info/storage.html

No server, eh? You could use something like Parse. Make a Parse object, set the key to the particular GitHub API URI, and set the value to something like this:
{
stored: <Date>,
value: <stringified JSON returned from GitHub API call>
}
Then when someone hits your client, first call Parse to see if you already have a cached version for that particular API call. If you don't, make the call to GitHub's API and then store the results on Parse (with stored set to the current DateTime so you can check for staleness later).
If Parse does have a cached version stored, check the stored value to see how old it is - if it is stale, make a fresh call to GitHub, and store the results back into Parse. Otherwise, just parse the JSON string from value and you're good to go.
This is assuming that you want individual caching control over the 14 GitHub API calls. If you don't, then just store the compiled calls into one object on Parse under a key like cache.

How do I know how long a user or a single IP address spent on a page of my site?

How do I know how long a user or a single IP address spent on a page of my site?
Can I use any event of JavaScript?

This is what Google Analytics is for. It keeps track of visitors to your website and reports statistics to you like such as length of visit. They even offer an API to help you get this information.

To implement inside your application
When a user loads a page, their "last page load time" is updated in the database.
A cron script runs every minute. It looks for users whose "last page load time" was in the last minute.
Increment the "time spent on site" for each user found.
In this way, I can keep track of users accurately to the minute. Just divide by 60 to get the hours.
Else
Google Analytics. You can check out it's features here.

I will go for a Redis installation that keeps tracks of a specific hash created by a combination of IP address, USERAGENT and login information (if you have it).
Save the stuff in Redis with a timestamp (use hashes or whatever you like) and then add a "farewell" timestamp when the user logs out or leaves the site for an external link or unloads the page.

Use Ajax to send keep-alive information to your backend. Save it as session data in your database.
The session data can save the current page the user is watching and was active last time. Send the keep-alive information using, for example, setInterval every x seconds (depending on resolution needs).
There is no need for cron...