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Change elements in webapp from value of variable in JS

I am making a webapp that has to be connected to an Arduino for its inputs so the contents of the webpage change.
I am currently not done yet, so I'm just trying it out with vars I have created and given random numbers. However, I am new to jQuery and cannot get it to work properly. For instance, I have the variable light = 100 and I want to make an if statement that if light > 100 the website displays the message: your plant is happy.
I don't know how to change the value with just the input of a variable, without having to click on any elements or anything.

I'm assuming that you are using Arduino hardware to detect the intensity of light and then use that information to update information on a webpage in real time.
One way to do this is:
Define the ranges for the plant state (eg. 'happy' 100+, 'okay' 80 to 100, 'sad' < 80, etc.)
Program your Arduino board to send the state to your server (eg: a python flask server) whenever the light intensity moves in and out of the set ranges.
Use Server Sent Events (SSE) or WebSockets to update the plant state into browser clients which are connected to your web server.

Prevent client from controlling data to server

I've been making a simple socket.io program where the players can move around in an environment, with the client sending the server events.
However, it seems that the client can use the console to falsify data and send it to the server (by using socket.emit()).
Is there a way to combat that, so that the server only accepts "real" data, or to prevent the client from sending false data?

Your server should always hold the state of the application, and have a list of all possible actions for each state.
For example, if your character can move on a map, the server should always keep the coordinate of the player. Let's say the player is at coordinate (x, y). The server will only allow messages that move the player to (x+1, y+1), (x-1, y+1), (x+1, y-1) or (x-1, y-1). Any other message should be discarded.
If it receives a message saying the player wants to move to (x+500, y+500), it should ignore it and potentially mark the player as a cheater and disconnect it.

Messages being delayed when using websockets

I have a program which is using the Websocket TCP: The client is an extension in Chrome and the server is an application written in C++.
When I send small data from the client to the server, it works fine. But when I send large amounts of data (e.g. a source html page), it will be slightly delayed.
For Example:
Client sends: 1,2,3
Server receives: 1,2
Client sends: 4
Server receives: 3
Client sends: 5
Server receives: 4
It's seems like it's a delay.
This is my code client:
var m_cWebsocket = new WebSocket("Servername");
if (m_cWebsocket == null) { return false; }
m_cWebsocket.onopen = onWebsocketOpen(m_cWebsocket); m_cWebsocket.onmessage = onWebsocketMessage;
m_cWebsocket.onerror = onWebsocketError;
m_cWebsocket.onclose = onWebsocketError;
I using m_cWebsocket.send(strMsg) to send data.
Server code
while (true) { recv(sSocket, szBufferTmp, 99990, 0); //recv(sSocket,
szBufferTmp, 99990, MSG_PEEK); //some process }

Since you haven't posted any code to show your implementation of the TCP server or client I can only speculate and try to explain what might be going on here.
That means the potential problems and solutions I outline below may or may not apply to you, but regardless this information should still be helpful to others who might find this question in the future.
TL;DR: (most likely) It's either the server is too slow, the server is not properly waiting for complete 'tcp packets' to be buffered, or the server doesn't know when to properly start and stop and is de-synching while it waits for what it thinks is a 'full packet' as defined by something like a buffer size.
It sounds to me like you are pushing data from the client either faster than the server the server can read, or more likely, the server is buffering a set number of bytes from the current TCP Stream and waiting for the buffer to fill before outputting additional data.
If you are sending this over localhost it's unlikely you are not close to limit of the stream though, and I would expect a server written in C++ would be able to keep up with the javascript client.
So this leads me to believe that the issue is in fact the stream buffer on the C++ side.
Now since the server has no way to know to what data you are sending and or how much of it you are sending, it is common for a TCP stream to utilize a stream buffer that contiguously reads data from the socket until either the buffer has filled to a known size, or until it sees a predefined 'stop character'. This would usually be something like a "line end" or \n character, sometimes \n\r (line feed, carriage feed) depending on your operating system.
Since you haven't specified how you are receiving your data, I'm going to just assume you created either a char or byte buffer of a certain size. I'm a pretty rusty on my C++ socket information so I might be wrong, but I do believe there is a default 'read timeout' on C++ tcp streams as well.
This means you are possibly running into 1 of 2 issues.
Situation 1) You are waiting until that byte/char buffer is filled before outputing it's data. Issue is that will act like a bus that only leaves the station when all seats are filled. If you don't fill all the seats, you server is just sitting and waiting until it gets more data to fill up fully and output your data.
Situation 2) You are running up against the socket read timeout and therefore the function is not getting all the data before outputting the data. This is like a bus that is running by the clock. Every 10 minutes that bus leaves the station, doesn't matter if that bus is full or empty, it's leaving and the next bus will pick up anyone who shows up late. In your case, the TCP stream isn't able to load 1, 2 and 3 onto a bus fast enough, so the bus leaves with just 1, 2 on it because after 20ms of not receiving data, the server is exiting from the function and outputing the data. On the next loop however, there is 3 waiting at the top of the stream buffer ready to get on the next bus out. The Stream will load 3, wait til those 20ms are finished, and then exit before repeating this loop.
I think it's more likely the first situation is occurring though, as I would expect the server to either start catching up, or falling further behind as the 2 servers either begin to sync together, or have internall TPC stream buffer fill up as the server falls further and further behind.
Main point here, you need some way to synchronize the client and the server connections. I would recommend sending a "start byte" and "End byte" to single when a message has begun and finished, so you don't exit the function too early.
Or send a start byte, followed by the packet size in bytes, then filling up the buffer until your buffer has the correct numbers of bytes. Additionally you could include an end byte as well for some basic error checking.
This is a pretty involved topic and hard to really give you a good answer without any code from you, but this should also help anyone in the future who might be having a similar issue.
EDIT I went back and re-read your question and noticed you said it was only with large amounts of data, so I think my original assumption was wrong, and it's more likely situation 2 because the client is sending the data to your server faster than the server can read it, and thus might be bottle necking the connection and the client is only able to send additional data once the server has emptied part of it's TCP stream buffer.
Think of it like a tube of of water. The socket (tube) can only accept (fill up) with so much data (water) before it's full. Once you let some water out the bottom though, you can fill it up a little bit more. The only reason it works for small files is that the file is too small to fill the entire tube.
Additional thoughts: You can see how I was approaching this problem in C# in this question: Continuously reading from serial port asynchronously properly
And another similar question I had previously (again in C#): How to use Task.WhenAny with ReadLineAsync to get data from any TcpClient
It's been awhile since I've played with TCP streams though, so my apologies in that I don't remember all the niche details and caveats of the protocal, but hopefully this information is enough to get you in the ball park for solving your problem.
Full disclaimer, it's been over 2 years since I last touched C++ TCP sockets, and have since worked with sockets/websockets in other languages (such as C# and JavaScript), so I may have some facts wrong about the behavior of C++ TCP sockets specifically, but the core information should still apply. If I got anything wrong, someone in the comments will most likely have the correct information.
Lastly, welcome to stack overflow!

NodeJS + Socket IO Sends Too Much Data

I am currently developing a game using NodeJS + SocketIO but is having problem with the amount of data being sent. The server currently sends about 600-800 kbps which is not good at all.
Here are my classes:
Shape
Pentagon
Square
Triangle
Entity
Player
Bullet
Every frame (60 fps), I update each of the classes and each class will have an updatePack that will be sent to the client. The updatePack is pretty simple, it only containts the object's id and coords.
At first, I thought everyone's game are like that (silly me). I looked into several simple games like agar.io, slither.io, diep.io, and rainingchain.com and found that they use < 100 kbps which made me realize that I am sending too much data.
Then I looked into compressing the data being sent. But then I found out that data are automatically compressed when sending in Socket.io
Here is how I send my data:
for(var i in list_containing_all_of_my_sockets){
var socket = list_containing_all_of_my_sockets[i];
data = set_data_function();
socket.emit('some message', data);
}
How can I make it send less data? Is there something that I missed?

Opinionated answer, considering a way games handle server-client traffic. This is not the answer:
Rendering is a presentation concern. Your server, which is the single source of truth about the game state, should only care about changing and advertising the game state. 60fps rendering is not a server concern and therefore the server shouldn't be sending 60 updates per second for all moving objects (you might as well be better of just rendering the whole thing on the server and sending it over as a video stream).
The client should know about the game state and know how to render the changing game state at 60fps. The server should only send either state changes or events that change state to the client. In the latter case the client would know how to apply those events to change the state in tandem with the state known to the server.
For example, the server could be just sending the updated movement vectors (or even the acting forces) for each object and the client could be calculating the coordinates of each object based on their currently known movement vectors + the elapsed time.

Maybe its better not to send data every frame, but instead send it only on some particular events (etc. collisions,deaths,spawns)

Whenever a message is send over a network it not only contains the actual data you want to send but also a lot of additional data for routing, error prevention and other stuff.
Since you're sending all your data in individual messages, you'll create these additional information for every single one of them.
So instead you should gather all data you need to send, save it into one object and send this one in a single message.

You could use arrays instead of objects to cut down some size (by omitting the keys). It will be harder to use the data later, but... you've got to make compromises.
Also, by the looks of it, Socket.IO can compress data too, so you can utilize that.
As for updating, I've made a few games using Node.js and Socket.IO. The process is the following:
Player with socket id player1 sends his data (let's say coordinates {x:5, y:5})
Server receives the data and saves it in an object containing all players' data:
{
"player1": {x:5, y:5},
"player2": ...
...
}
Server sends that object to player1.
player1 receives the object and uses the data to visualize the other players.
Basically, a player receives data only after he has sent his own. This way, if his browser crashes, you don't bombard him with data. Otherwise, if you've sent 15 updates while the user's browser hanged, he needs more time to process these 15 updates. During that time, you send even more updates, so the browser needs even more time to process them. This snowballs into a disaster.
When a player receives data only after sending his own, you ensure that:
The sending player gets the other players' data immediately, meaning that he doesn't wait for the server's 60 times-per-second update.
If the player's browser crashes, he no longer sends data and therefore no longer receives it since he can't visualize it anyway.

How would you create an auto-updating newsfeed without a reload?

How would I go around creating an auto-updating newsfeed? I was going to use NodeJS, but someone told me that wouldn't work when I got into the thousands of users. Right now, I have it so that you can post text to the newsfeed, and it will save into a mysql database. Then, whenever you load the page, it will display all the posts from that database. The problem with this is that you have to reload the page everytime there is an update. I was going to use this to tell the nodejs server someone posted an update...
index.html
function sendPost(name,cont) {
socket.emit("newPost", name, cont);
}
app.js
socket.on("newPost", function (name,cont) {
/* Adding the post to a database
* Then calling an event to say a new post was created
* and emit a new signal with the new data */
});
But that won't work for a ton of people. Does anyone have any suggestions for where I should start, the api's and/or programs I would need to use?

You're on the right track. Build a route on your Node webserver that will cause it to fetch a newspost and broadcast to all connected clients. Then, just fire the request to Node.
On the Node-to-client front, you'll need to learn how to do long polling. It's rather easy - you let a client connect and do not end the response until a message goes through to it. You handle this through event handlers (Postal.JS is worth picking up for this).
The AJAX part is straightforward. $.get("your/node/url").then(function(d) { }); works out of the box. When it comes back (either success or failure), relaunch it. Set its timeout to 60 seconds or so, and end the response on the node front the moment one event targetted it.
This is how most sites do it. The problem with websockets is that, right now, they're a bit of a black sheep due to old IE versions not supporting them. Consider long polling instead if you can afford it.
(Psst. Whoever told you that Node wouldn't work in the thousands of users is talking through their asses. If anything, Node is more adapted to large concurrency than PHP due to the fact that a connection on Node takes almost nothing to keep alive due to the event-driven nature of Node. Don't listen to naysayers.)