Do I need to cache localStorage? - javascript

If I'm looping and referencing a variable that's kept in localStorage, should I create a locally scoped variable (outside the loop) and set it equal to the localStorage variable for performance reasons?

This may turn a micro-optimization unless you access it REALLY often and it takes the greater part of your loop.

If you are referencing the same local storage value multiple times within a function, then assign it to a local variable for the duration of that function. This is no different than any other value that takes some work to retrieve (like the value of an input field in the DOM). If you need the value multiple times within the same function, then put it's value in a local variable and use it from there. Your code will probably be more compact and execute faster too.
There should be no reason to cache it globally in a persistent global variable as it's already globally accessible from local storage so there's really no reason to add a new global for it. Just retrieve the value in each function that you need it in. The only exception I could imagine to this would be a micro-performance-optimization in a rare circumstance. Generally, it's better not to make your own global copy of things that are already globally available.

No, you won't get that much performance. Ofc if you'r querying too many times, you should.

Related

Persistant variable with Gmail scripting (Google Apps Script) [duplicate]

In a Google spreadsheet using the Script Editor, I do function calls, but I am not quite sure if the best way to store persistant data (data that I will continue to use) is to use global variables (using objects, arrays, strings), or there is a better way to store data.
I don't want to use cells which could be another way.
Another question, is it possible to create (pseudo) classes in this environment? Best way?
Both ScriptProperties and ScriptDB are deprecated.
Instead, you should be using the new class PropertiesService which is split into three sections of narrowing scope:
Document - Gets a property store that all users can access within the current document, if the script is published as an add-on.
Script - Gets a property store that all users can access, but only within this script.
User - Gets a property store that only the current user can access, and only within this script.
Here's an example persisting a user property across calls:
var properties = PropertiesService.getScriptProperties();
function saveValue(lastDate) {
properties.setProperty('lastCalled', lastDate);
}
function getValue() {
return properties.getProperty('lastCalled');
}
The script execution environment is stateless, so you cannot access local variables from previous runs, but you can store getScriptProperties() in a local variable because it will be re-run for each return trip to the server so it can be called in either method.
If you need to store something on a more temporary basis, you can use the CacheService API
Persistent data can be stored using the Class ScriptProperties:
http://code.google.com/googleapps/appsscript/class_scriptproperties.html
All values are stored as a string and will have to be converted back with the likes or parsInt or parseFloat when they are retrieved.
JSON objects can also be stored in this manner.
My experience has been that every query to retrieve or store values takes a long time. At the very least, I would cache the information in your javascript code as much as possible when it is safe. My scripts always execute all at once, so I don't need to keep global variables as I simply pass the retrieved data arrays around, manipulate them, and finally store them back in one fell swoop. If I needed persistence across script invocations and I didn't care about dropping intermediate values on close of the webpage, then I'd use globals. Clearly you have to think about what happens if your script is stopped in the middle and you haven't yet stored the values back to Google.

Global variables VS localStorage in web app and memory issues

I'm wondering which would be better practice. Polluting the global namespace with global variables for intra-session persistence or using localStorage instead?
So in other words set a global variable on launch, change its value in a function when required and reference it in a third function, or use localStorage.setItem then localStorage.removeItem when the value is no longer needed?
Will doing either one increase memory efficiency?
LocalStorage is primarily for persistent data across sessions.
In your case, as your looking for an intra-session persistence, global variables have clear advantages.
I will start with cons of global variables first.
It uses the global namespace, any third party js code can manipulate it
A page refresh can wipe off your data
Well, that's it. If we consider the cons of LocalStorage, the list will raise your eyebrows.
set and get are slow and can be a performance bottleneck for large datasets
only strings are allowed; you may have to serialize your data before setting
I would surely vote up for LocalStorage if your use case involved inter-session storage. However, in your scenario, the only benefit you see is the removeItem function for which you have the delete counterpart for global variables.
This article may be helpful: http://www.sitepoint.com/html5-browser-storage-past-present-future/
As now consider to using DI in frameworks like Angular.

Javascript performance: Variable vs Large object's attribute

I am building a number of HTML5 games and I am not sure about one thing in javascript.
When dealing with a large object (with a lot of attributes and methods), is it somehow different, if I store the attribute value in a variable?
Say I have to check for some value in application.data.setings.foo.bar multiple times per second. Should I store it in a variable fooBar? If I understand it correctly, the variable would be just a reference, so it shouldn't matter.
so: Should you store values of large objects' attributes in variables?
This diagram give you your answer. http://oreilly.com/server-administration/excerpts/even-faster-websites/writing-efficient-javascript.html#access_times_for_object_properties_by_de
I believe what you are asking is:
If you have a variable called:
application.data.settings.foo.bar = { some: 'value' }
Is it more performant to access it by using application.data.settings.foo.bar - or to instead say: var bar = application.data.settings.foo.bar and then refer to bar.
My guess is that the assignment to variable is probably slightly more performant - simply because there are less steps for the interpreter to take in order to access the relevant item. If you use application.data.settings.foo.bar - the interpreter has to utilize the reference to the application object, then reference it's data property, then reference its setting property, then reference its foo property, then reference its bar property. This is 5 steps.
If you reference it into a local variable for access - you are still hitting the same object, but you are hitting it directly on each reference.
At the end of the day, however, this is unlikely to be a performance enhancement which is hugely noticeable, unless you are doing a lot of heavy, rapid-access looping or something similar.
For numbers you may see a performance improvement if you place it in a variable

What are some of the problems of "Implied Global variables"?

JavaScript: The Good Parts defines these kinds of declarations as bad:
foo = value;
The book says "JavaScript’s policy of making forgotten variables global creates
bugs that can be very difficult to find."
What are some of the problems of these implied global variables other than the usual dangers of typical global variables?
As discussed in the comments on this answer, setting certain values can have unexpected consequences.
In Javascript, this is more likely because setting a global variable actually means setting a property of the window object. For instance:
function foo (input) {
top = 45;
return top * input;
}
foo(5);
This returns NaN because you can't set window.top and multiplying a window object doesn't work. Changing it to var top = 45 works.
Other values that you can't change include document. Furthermore, there are other global variables that, when set, do exciting things. For instance, setting window.status updates the browser's status bar value and window.location goes to a new location.
Finally, if you update some values, you may lose some functionality. If, for instance, you set window.frames to a string, for instance, you can't use window.frames[0] to access a frame.
Global variable make it very difficult to isolate your code, and to reuse it in new contexts.
Point #1:
If you have a Javascript object that relies on a global var. you will not be able to create several instances of this object within your app because each instance will change the value of the global thereby overwriting data the was previously written by the another instance.
(Unless of course this variable holds a value that is common to all instances - but more often than not you'll discover that such an assumption is wrong).
Point #2:
Globals make it hard to take existing pieces of code and reuse them in new apps. Suppose you have a set of functions defined in one file and you want to use them in some other file (in another app). So you extract them to a new file and have that file included in the new app. If these function rely on a global your 2nd app will fail at runtime because the global variable is not there. The dependency on globals is not visible in the code so forgetting these variables (when moving functions to new files) is a likely danger.
They're global variables, so yes, all of the "usual dangers" apply. The main thing that distinguishes them from global variables in other languages is that:
You don't explicitly declare them in a global scope. If you mistakenly omit var in a variable declaration, you've accidentally declared a global variable. JavaScript makes it a little too easy to unintentionally declare global variables; contrast this with Scheme, which raises an error if a variable is not defined in the global scope.
Global variables, at least in the browser, are aliased by window[variable_name]. This is potentially worrisome. For example, some of your code might access window['foo'] (with the intention of accessing a global variable). Then, if you accidentally type foo instead of var foo elsewhere in the program, you have declared a reference to window['foo'], which you meant to keep separate.
One issue is that you may be trampling on already defined variables and not know it, causing weird side effects in other parts of the code that can be a bear to track down.
Another is that it is just sloppy code. You should not be creating variable with more scope than they need since at the very least it keeps more variables in memory and at worst it can create data scenarios you didn't intend.
The bottom line is that when you do that you don't know for sure that you are not messing up other functions that use a global variable of the same name. Sometimes it isn't even your fault, a lazy programmer of another plugin left something global that was meant to have scope inside of a function. So it is a very practical safeguard for writing better and less buggy code.
The problems of typical global variables is that they are, well, global - there is no scope to enclose them, and any code that you are executing / interacting with (such as a library that you call down the road) could modify the variable without a warning.
However, these problems are compounded in Javascript by two things:
You can define a global variable anywhere - the only requirement for that is to forget the var keyword.
It is extremely easy to define a global variable when you had no intent to do so. That is the problem that "implied" globals have over "typical" globals - you will create them without even knowing you did.
In a reasonably-designed language that includes truly global variables (ok, so not that reasonably-designed), you would have a limited handful of places to define globals, and it would require a special keyword to do so.

Javascript Function Scope

This is perhaps a dumb question, but I am new to Javascript and desperate for help.
If the Javascript engine will look for global variables outside of a function, then what is the point of passing parameters to it? What do you gain?
I understand that global variables are generally frowned upon, but I still don't understand the purpose of passing variables. Does it have something to do with data encapsulation?
There's a few magic words that are used by programmers to describe different kinds of functions. Here's a few:
Re-entrant
ThreadSafe
Referentially Transparent
Idempotent
Pure
Side-Effects
You can look some of them up if you want a headache. The point is that Computer science and engineering progress has always been about reducing complexity. We have spent quite a lot of time thinking about the best way to write a function to achieve that goal. Hopefully, you can stuff tiny bits of your program into your head at a time, and understand those bits, without having to also understand the overall functioning of the entire program simultaneously, or the detailed implementation of the insides of all the other functions. A function that uses global variables can't do that very well because:
You can't guarantee that the global variables exist
You can't guarantee that the global variables are what you think they are
You can't guarantee that other parts of the program haven't modified those variables in a way you didn't expect.
You can't easily generalise to use the function multiple times on multiple sets of variables.
You can't easily verify that the function works as advertised without first setting up the function's external environment and its dependencies.
If the global variables have changed in a way you didn't expect, it's really hard to track down which part of the program is the culprit. It could be any of 500 different functions that write to that variable!
On the other hand, if you explicitly pass in all the data a function needs to operate, and explicitly return all the results:
If something goes wrong with any of those variables, it's easy to find the source of the problem
It's easier to add code to verify the "domain" of your inputs. Is it really a string? Is it over a certain length, is it under a certain length? Is it a positive number? is it whole, or fractional? All these assumptions that your code needs to operate correctly can be explicit at the start of the function, instead of just crossing your fingers and hoping nothing goes wrong.
It's easier to guess what a particular function will actually do, if its output depends only on its input.
a function's parameters are not dependant on the naming of any external variables.
And other advantages.
if you were only going to use global variables that the functions worked on then you'd always have to know the inner workings of the functions and what your global variable names had to be for them to work.
also, something like Math.abs(n) would be hard to call twice in one line if using global variables.
Functions are reusable components of your code, that executes a particular snippet on the provided variable exhibiting varying behavior.
Encapsulation comes from being Object Oriented. Functions are more for giving structure to your program.
Also, you shouldn't undermine the execution time of a method, if the variable it access exists in the context rather than being global.
If you don't need parameters to be passed to functions, then, you really don't need functions.
Functions are usually (and should be) used to provide code re-use -- use the same function on different variables. If a function accesses global variables, then every time I use it it will perform the same action. If I pass parameters, I can make it perform a different action (based on those different parameters) every time I use it.
One of the main benefits is that it keeps all the information the function needs, nearby. It becomes possible to look at just the function itself and understand what its input is, what it does, and what its output will be. If you are using global variables instead of passing arguments to the function, you’ll have to look all over your code to locate the data on which the function operates.
That’s just one benefit, of many, but an easy one to understand.
Global variables (in any language) can sometimes become stale. If there is a chance of this it is good to declare, initialise and use them locally. You have to be able to trust what you are using.
Similarly, if something/someone can update your global variables then you have to be able to trust the outcome of what will happen whenyou use them.
Global variables are not always needed by everything so why keep them hanging around?
That said, variables global to a namesapce can be useful especially if you are using something like jquery selectors and you want to cache for performance sake.
Is this really a javascript question? I haven't encountered a language that doesn't have some form of global variables yet.

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