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What's the performance impact of global and instance methods in vue.js

I was discussing a possible solution with a workmate and, we agree that expose a global function would be the easiest and fast way, but we disagree about the approach to do that. He argues that add many instance methods could cause performance loss and recommend to add a global function instead. I think that does not have a difference because, in both approaches, global or instance, the method would be exposed and consequently load into memory. However, I've not a deep understanding of how Vue works and, I hope someone to explain in more detail the performance impacts in both ways.
With global function I mean, directly into vue like that
Vue.myGlobalMethod = function () {}
And instace like that:
Vue.prototype.$myMethod = function (methodOptions) {}

Is using `var` to declare global scope as bad as using `global`

I've learned that using globals is a bad idea in Javascript because there's the risk of collisions with dependencies.
Can I use var at the global scope as an alternative to this? I want to avoid the pitfalls of globals, but doing this seems much easier than passing along all my custom objects as parameters to functions. I have objects which are partially defined in many different files.

Can I use var at the global scope as an alternative to this?
var in global scope creates a global variable. So it's exactly the same and not in any way better.
If you are working with Node, there isn't really a need for globals. Every module should require all of its dependencies.

Is there any reason to define module.exports using an IIFE?

My team doesn't have any experienced JS developers, but we are writing a library in Node and got a suggestion from a real JS developer that "We should make the js more modular - not to pollute the global namespace and to make it more readable to new-comers", and told us to do the following:
module.exports = (function(){
return {
nameToExpose: functionToExpose
...
};
})();
rather than
module.exports.nameToExpose = functionToExpose;
What's the point of this, if any? The latter does not make any local declarations that would be scoped by the IIFE, and even if it did, they would be local to the module file and not global to the whole program that require()s it.
Some Googling and poking about this site does not turn up any answers on this particular question, though there are many other explanations of IIFEs that I have read (and which are summarized in the above comment). Some testing certainly reveals that the latter does not actually put functionToExpose in the global namespace, though its original name is recorded in the function type itself.

Pretty much no difference. The whole idea of Node.js, using require, having modules, etc., is specifically to separate concerns. I'd say (cautiously) that if you're doing it right, you shouldn't be needing to worry about "polluting" any sort of global scope. Anything within module.exports lives in that module.
When you're dealing with front-end stuff, that's when the global scope becomes something of a concern, because if a function or whatever isn't scoped (i.e., in an IIFE, or other function block), it has access to the global window object, and everything else has access to that function.
a real JS developer
Calling someone that is a red flag.
not to pollute the global namespace and to make it more readable to new-comers
If you're modularizing your code correctly, that shouldn't be a concern. There's a time and a place for IIFEs, but I see no reason why wrapping everything in an IIFE, which is already inside of a module, would somehow magically make the code "more modular" or any more readable to "new comers" than by simply using Node.js like it was designed:
module.exports = function() { ... } // whatever
and even if it did, they would be local to the module file and not global to the whole program that require()s it.
You are correct. I'd take whatever he's saying with a grain of salt. Maybe he knows of some specific use-cases where his approach has been helpful to him in the past, so I'd ask him specifically about that to see what he says. Other than that, I feel like you're on the right track.

The reason to maybe sometimes do this is because if you don't, then any variables you need for the module.exports object have to be scoped to the entire file.
Consider these two ways.
Without IIFE.
var foo = 'la' + 'la'; // some computed value
//
// ... lots of program code here ...
//
module.exports = {
foo : foo,
};
With IIFE.
//
// ... lots of program code here ...
//
module.exports = (function () {
var foo = 'la' + 'la'; // some computed value
return {
foo : foo
}
}());
In the first example, two problems arise.
Your variables (like foo) are created quite far away from where they are used to export a value from the module. This can reduce clarity. Sure, you can declare a variable after the program code, but it still has the same scope (and vars are hoisted). Plus, general best practice is to declare all your variables up front, and not doing so is a tradeoff to consider.
The program code can mess around with your variables, intentionally or accidentally, which complicates things and is undesirable unless you need that (sometimes you do).
The second example eliminates these problems by having a private scope for that area of the file. You can still use variables that are scoped to the entire file, but in cases where you don't need that, you can have variables that are easier to read and understand.
Often times we program for humans, not machines. This is an example of optimizing for the former.
Update:
In modern versions of JavaScript, const and let are probably better solutions to the problems this pattern aims to solve. With them, you can define variables in a way that will throw errors if you make the same mistakes the IIFE is trying to protect you from.
//
// ... lots of program code here ...
//
const foo = 'la' + 'la'; // some computed value
module.exports = {
foo : foo,
};
In the above example, if the program code uses foo, it will crash with a ReferenceError, because of the Temporal Dead Zone, as opposed to receiving undefined as a var would. This is great because now you must explicitly move the declaration of foo to an earlier place in the code if its use was intentional, or otherwise fix the code.

It's a good idea. There's a lot about javascript that seems to push the envelope of how 'modern programming' is done. Maybe one of those is the modular system of CommonJS. But the evolution in javascript has always been along the lines of the classical object oriented paradigms of software development. It most likely always will be.
Javascript may do things differently, but it doesn't do different things... if that makes sense.
It's always a good idea to protect scope and objects from excessive mutations.
https://medium.com/backticks-tildes/the-s-o-l-i-d-principles-in-pictures-b34ce2f1e898

Javascript optimization...global variables

I am making a webapp. I have a fairly basic question about javascript performance. Sometimes I need a global variable to store information that is used the entire time the website is open.
An example is a variable called needs_saved. It is true or false to say whether the page needs saved. I might have another variable called is_ie, ie_version, or space_remaining.
These are all variable that I need in various functions throughout the app.
Now, I know global variables are bad because they require the browser to search each level of function scope. But, I don't know if there is any better way to store values that are needed throughout the program.
I know I could create a global object called 'program_status' and give it the properties is_ie, ie_version, etc... But is this any better since it would first have to find my program_status object (stored as a global variable), and then the internal property?
Maybe I'm overthinking this.
Thanks

You have nothing to worry about.
The performance impact of a global variable is minute.
Global variables are discouraged because they can make code harder to maintain.
In your case, they won't.

The reason global variable use should be kept to a minimum is because the global namespace gets polluted when there's a lot of them, and there's a good chance of a clash if your program needs to use some 3rd party libraries which also create their own globals.
Creating a single object to hold all of your global state is a good idea since it limits the number of identifiers you need to reserve at the global level.
To solve performance problems, you can then create a local reference to that object in any scope where you need to access it multiple times:
So instead of
if (globalState.isIe) { alert(globalState.ieMessage); }
you can do
var state = globalState;
if (state.isIe) { alert(state.ieMessage); }
You don't need to do this if you only access the state object once. In any case, even if you never do this, the performance penalties will be negligible.

If you're worried about performance, code something clean then run a profiler on it to optimize. I know both Safari and Google Chrome have one, and it's pretty sure Firebugs includes one for Firefox too. Heck, even Internet Explorer 8 has one.

Why is it bad to make elements global variables in Javascript?

I've heard that it's not a good idea to make elements global in JavaScript. I don't understand why. Is it something IE can't handle?
For example:
div = getElementById('topbar');

I don't think that's an implementation issue, but more a good vs bad practice issue. Usually global * is bad practice and should be avoided (global variables and so on) since you never really know how the scope of the project will evolve and how your file will be included.
I'm not a big JS freak so I won't be able to give you the specifics on exactly why JS events are bad but Christian Heilmann talks about JS best practices here, you could take a look. Also try googling "JS best practices"
Edit: Wikipedia about global variables, that could also apply to your problem :
[global variables] are usually
considered bad practice precisely
because of their nonlocality: a global
variable can potentially be modified
from anywhere, (unless they reside in
protected memory) and any part of the
program may depend on it. A global
variable therefore has an unlimited
potential for creating mutual
dependencies, and adding mutual
dependencies increases complexity. See
Action at a distance. However, in a
few cases, global variables can be
suitable for use. For example, they
can be used to avoid having to pass
frequently-used variables continuously
throughout several functions.
via http://en.wikipedia.org/wiki/Global_variable

Is it something IE can't handle?
No it is not an IE thing. You can never assume that your code will be the only script used in the document. So it is important that you make sure your code does not have global function or variable names that other scripts can override.
Refer to Play Well With Others for examples.

I assume by "events" you mean the event-handling JavaScript (functions).
In general, it's bad to use more than one global variable in JS. (It's impossible not to use at least one if you're storing any data for future use.) That's because it runs into the same problem as all namespacing tries to solve - what if you wrote a method doSomething() and someone else wrote a method called doSomething()?
The best way to get around this is to make a global variable that is an object to hold all of your data and functions. For example:
var MyStuff = {};
MyStuff.counter = 0;
MyStuff.eventHandler = function() { ... };
MyStuff.doSomething = function() { ... };
// Later, when you want to call doSomething()...
MyStuff.doSomething();
This way, you're minimally polluting the global namespace; you only need worry that someone else uses your global variable.
Of course, none of this is a problem if your code will never play with anyone else's... but this sort of thinking will bite you in the ass later if you ever do end up using someone else's code. As long as everyone plays nice in terms of JS global names, all code can get along.

There shouldn't be any problem using global variables in your code as long as you are wrapping them inside a uniqe namespase/object (to avoid collision with scripts that are not yours)
the main adventage of using global variable in javascript derives from the fact that javascript is not a strong type language. there for, if you pass somes complex objects as arguments to a function, you will probebly lose all the intellisence for those objects (inside the function scope.)
while using global objects insteads, will preserve that intellisence.
I personally find that very usfull and it certainly have place in my code.
(of course, one should alwayse make the right balance between locales and globals variables)