I've been dipping my feet into javascript and more specifically node.js but I'm having trouble identifying required parameters for callbacks.
For example, when creating a route through Express, I can have the following
app.get('/', function() {
console.log('this is a route');
});
Which will execute without giving me any trouble. However, having seen multiple examples, I know that I probably want to have something more along the lines of
app.get('/', function(req, res) {
res.render('index');
});
But without having seen examples or documentation (which is sometimes just a couple unclear examples) is there a consistent way to determine what parameters a callback is expected to have?
I hope I've been clear.
Without documentation, or inspecting the source of the function executing the callback, you wont easily know.
However, you can intercept them with some exploratory code and see what you get:
app.get('/', function() {
console.log(arguments);
});
The arguments keyword here is the list of arguments passed to the callback function, so this will let you see what you got. If it tell you something is a Express.Request or something, this at least lets you know what to try to find in the docs.
But outside of standard javascript, using typescript or flow helps with this since it adds static types to javascript. If this function is typed, then your editor will then know arguments the callback function expects and can help you fill them in.
Since you're using Express, the documentation is pretty clear - It depends on your route parameters and whether or not you're using middleware. There is no hard and fast rule, it genuinely depends on your route's function.
Your first example "works" because you're only printing to the console, but without the res response object you'll notice that that the request response returns nothing.
Start with req and res for each and expand as needed.
I have been doing extensive research on google but I still cannot find my answer. From what I understand, a helper function is a function which helps reduce the complexity of some computation by providing a name for a part of the computation and the ability to take arguments. From the few codes I've seen, so do the other two functions...
I'm going to try to keep the terminologies and buzzwords down seeing that most beginners struggle to grasp new concepts because they go down a terminology rabbit hole.
Callback functions contain code you want an underlying subsystem to execute after it completes a specific task that is not sequential in nature. The subsystem is usually an abstraction for something happening under the hood and exposes an API for you to use which accepts the function. Think of it like sending a one-way message to someone/something to help you complete a task with no guaranteed ETA and then leaving a number/address they could reach you when the task is done. This is a concept that is natural to languages that accept functions as data.
Helper functions basically contain required "detour" code that helps with separating concerns. A function reads better when you make it do one thing well. If your function is doing too much then you might want to carefully think about what you are trying to accomplish and the steps to get you there. For example, say you have some code that needs to calculate the price for delivering a package from point A to B, part of that task also involves calculating the distance between two points. You don't want business code mixed up with your cryptic Haversine formula code, do you? In this case, your Haversine formula calculation can be a helper function. Helpers can also help with code reuse if they need to be used in more than one place in your project.
Wrapper functions let you wrap complex or foreign code in order to achieve some goal. That goal could be to expose a different set of interface, hide complexity or execute some code before and/or after calling the underlying function by way of proxy. When one function A calls another function B, A is said to have wrapped B.
Hope that helps.
What I need:
asyncFunc(..., function cb() {
console.log("async function finished");
});
console.log("AFTER async function finished");
I am aware that the title and this example may trigger some strong responses regarding proper async programming in JS, but if you don't believe me that this is at least an ok solution, I added a description why I need it this way, but I would also be quite happy with something "better".
My project uses the newest version of typescript, therefore a possible solution could use async/await, generators or anything else from the newer concepts.
Why I need it:
I know that this is wrong in most contexts and I may have overlooked some options in the specifics, but I think that's the best solution.
I am developing a library that runs before the actual project (Angular, React, ...) and is based on #Decorators. This library is only used in development, therefore the added startup time - which is introduced by a blocking solution - is acceptable. I have a class decorator that sets up a data structure which then is used inside other decorators. The data structure setup is asynchronous (asyncFunc from above) and I need to halt further execution until the data structure is set up, otherwise it is not ready when the decorators that use this data are called. Since decorators are decoupled and have a predefined signature and behavior, I am not aware of any way to make them asynchronous or dependent on another function.
thanks
I have a web application based on apache. php, js and jquery. All works fine.
On the client side there is a small library in JS/jquery, offering some generic list handling methods. In the past I used callbacks to handle those few issues where those methods had to behave slightly different. That way I can reuse methods like list handling, dialog handling and stuff for different part of the application. However recently the number of callbacks I had to hand through when stepping into the library grew and I am trying a redesign:
Instead of specifying all callbacks as function arguments I created a central catalog object in the library. Each module of the application registers its own variant of callbacks into that catalog upon initialization. At runtime the methods lookup the required callbacks in that catalog instead of expecting it specified in their list of arguments. This cleans up things quite a lot.
However I have one thing I still cannot get rid of: I require a single argument (I call it context, mode might be another term) that is used by the methods to lookup the required callback in the catalog. This context has to be handed through to all methods. Certainly better than all sorts of different callbacks being specified everywhere, but I wonder if I can get rid of that last one to.
But where do I specify that context, if not as method argument ? I am pretty new to JS and jquery, so I failed to find an approach for this. Apparently I don't want to use global vars, and to be frank I doubt that I can simply store a context in a single variable anyway, since because of all the event handlers and external influences methods might be called in different contexts at the same time, or at least interleaving. So I guess I need something closer to a function stack. Maybe I can simply push a context object to the stack and read that from within the layers of the library that need to know ? The object would be removed when I leave the library again. Certainly other approaches exist too.
Here are so many experienced coders how certainly can give a newbie like a short hint, a starting point that leads to an idea, how to implement this. How is such thing 'usually' done ?
I tried round a while, exploring the arguments.callee.caller hierarchy. I thought maybe I could set a prototype member inside a calling function, then, when execution steps further down I could simply traverse the call stack upwards until I find a caller holding such property and use that value as context.
However I also saw the ongoing discussions that reveal two things: 1.) arguments.callee appears to be depreciated and 2.) it appears to be really expensive. So that is a no go.
I also read about the Function.caller alternative (which appears not to be depreciated and much more efficient, however until now I failed to explore that trail...
As written currently passing the context/mode down simply works by specifying an additional argument in the function calls. It carries a unique string that is used as a key when consulting the catalog. So something like this (not copied, but written as primitive example):
<!-- callbacks -->
callback_inner_task_base:function(arg1,arg2){
// do something with args
}
callback_inner_task_spec:function(arg1,arg2){
// do something with args
}
<!-- catalog -->
Catalog.Callback:function(context,slot){
// some plausibility checks...
return Catalog[context][slot];
}
Catalog.base.slot=callback_inner_task_base;
Catalog.spec.slot=callback_inner_task_spec;
<!-- callee -->
do_something:function(arg1,arg2,context){
...
// callback as taken from the catalog
Catalog.Callback(callback,'inner_task')(arg1,arg2);
...
}
<!-- caller -->
init:function(...){
...
do_something('thing-1',thing-2','base');
do_something('thing-1',thing-2','spec');
...
}
But where do I specify that context, if not as method argument ?
Use a function property, such as Catalog.Callback.context
Use a monad
I'm new-ish to JavaScript. I understand many of the concepts of the language, I've been reading up on the prototype inheritance model, and I'm whetting my whistle with more and more interactive front-end stuff. It's an interesting language, but I'm always a bit turned off by the callback spaghetti that is typical of many non-trivial interaction models.
Something that has always seemed strange to me is that in spite of the readability nightmare that is a nest of JavaScript nested callbacks, the one thing that I very rarely see in many examples and tutorials is the use of predefined named functions as callback arguments. I'm a Java programmer by day, and discarding the stereotypical jabs about Enterprise-y names for units of code one of the things I've come to enjoy about working in a language with a strong selection of featureful IDE's is that using meaningful, if long, names can make the intent and meaning of code much clearer without making it more difficult to actually be productive. So why not use the same approach when writing JavaScript code?
Giving it thought, I can come up with arguments that are both for and against this idea, but my naivety and newness to the language impairs me from reaching any conclusions as to why this would be good at a technical level.
Pros:
Flexibility. An asynchronous function with a callback parameter could be reached by one of many different code paths and it could be harried to have to write a named function to account for every single possible edge case.
Speed. It plays heavily in to the hacker mentality. Bolt things on to it until it works.
Everyone else is doing it
Smaller file sizes, even if trivially so, but every bit counts on the web.
Simpler AST? I would assume that anonymous functions are generated at runtime and so the JIT won't muck about with mapping the name to instructions, but I'm just guessing at this point.
Quicker dispatching? Not sure about this one either. Guessing again.
Cons:
It's hideous and unreadable
It adds to the confusion when you're nested nuts deep in a swamp of callbacks (which, to be fair, probably means you're writing poorly constructed code to begin with, but it's quite common).
For someone without a functional background it can be a bizarre concept to grok
With so many modern browsers showing the ability to execute JavaScript code much faster than before, I'm failing to see how any trivial sort of performance gain one might get out using anonymous callbacks would be a necessity. It seems that, if you are in a situation where using a named function is feasible (predictable behavior and path of execution) then there would be no reason not to.
So are there any technical reasons or gotchas that I'm not aware of that makes this practice so commonplace for a reason?
I use anonymous functions for three reasons:
If no name is needed because the function is only ever called in one place, then why add a name to whatever namespace you're in.
Anonymous functions are declared inline and inline functions have advantages in that they can access variables in the parent scopes. Yes, you can put a name on an anonymous function, but that's usually pointless if it's declared inline. So inline has a significant advantage and if you're doing inline, there's little reason to put a name on it.
The code seems more self-contained and readable when handlers are defined right inside the code that's calling them. You can read the code in almost sequential fashion rather than having to go find the function with that name.
I do try to avoid deep nesting of anonymous functions because that can be hairy to understand and read. Usually when that happens, there's a better way to structure the code (sometimes with a loop, sometimes with a data table, etc...) and named functions isn't usually the solution there either.
I guess I'd add that if a callback starts to get more than about 15-20 lines long and it doesn't need direct access to variables in the parent scope, I would be tempted to give it a name and break it out into it's own named function declared elsewhere. There is definitely a readability point here where a non-trivial function that gets long is just more maintainable if it's put in its own named unit. But, most callbacks I end up with are not that long and I find it more readable to keep them inline.
I prefer named functions myself, but for me it comes down to one question:
Will I use this function anywhere else?
If the answer is yes, I name/define it. If not, pass it as an anonymous function.
If you only use it once, it doesn't make sense to crowd the global namespace with it. In today's complex front-ends, the number of named functions that could have been anonymous grows quickly (easily over 1000 on really intricate designs), resulting in (relatively) large performance gains by preferring anonymous functions.
However, code maintainability is also extremely important. Each situation is different. If you're not writing a lot of these functions to begin with, there's no harm in doing it either way. It's really up to your preference.
Another note about names. Getting in the habit of defining long names will really hurt your file size. Take the following example.
Assume both of these functions do the same thing:
function addTimes(time1, time2)
{
// return time1 + time2;
}
function addTwoTimesIn24HourFormat(time1, time2)
{
// return time1 + time2;
}
The second tells you exactly what it does in the name. The first is more ambiguous. However, there are 17 characters of difference in the name. Say the function is called 8 times throughout the code, that's 153 extra bytes your code didn't need to have. Not colossal, but if it's a habit, extrapolating that to 10s or even 100s of functions will easily mean a few KB of difference in the download.
Again however, maintainability needs to be weighed against the benefits of performance. This is the pain of dealing with a scripted language.
A bit late to the party, but some not yet mentioned aspects to functions, anonymous or otherwise...
Anon funcs are not easily referred to in humanoid conversations about code, amongst a team. E.g., "Joe, could you explain what the algorithm does, within that function. ... Which one? The 17th anonymous function within the fooApp function. ... No, not that one! The 17th one!"
Anon funcs are anonymous to the debugger as well. (duh!) Therefore, the debugger stack trace will generally just show a question mark or similar, making it less useful when you have set multiple breakpoints. You hit the breakpoint, but find yourself scrolling the debug window up/down to figure out where the hell you are in your program, because hey, question mark function just doesn't do it!
Concerns about polluting the global namespace are valid, but easily remedied by naming your functions as nodes within your own root object, like "myFooApp.happyFunc = function ( ... ) { ... }; ".
Functions that are available in the global namespace, or as nodes in your root object like above, can be invoked from the debugger directly, during development and debug. E.g., at the console command line, do "myFooApp.happyFunc(42)". This is an extremely powerful ability that does not exist (natively) in compiled programming languages. Try that with an anon func.
Anon funcs can be made more readable by assigning them to a var, and then passing the var as the callback (instead of inlining). E.g.:
var funky = function ( ... ) { ... };
jQuery('#otis').click(funky);
Using the above approach, you could potentially group several anon funcs at the top of the parental func, then below that, the meat of sequential statements becomes much tighter grouped, and easier to read.
Anonymous functions are useful because they help you control which functions are exposed.
More Detail: If there is no name, you can't reassign it or tamper with it anywhere but the exact place it was created. A good rule of thumb is, if you don't need to re-use this function anywhere, it's a good idea to consider if an anonymous function would be better to prevent getting tampered with anywhere.
Example:
If you're working on a big project with a lot of people, what if you have a function inside of a bigger function and you name it something? That means anyone working with you and also editing code in the bigger function can do stuff to that smaller function at any time. What if you named it "add" for instance, and someone reassigned "add" to a number instead inside the same scope? Then the whole thing breaks!
PS -I know this is a very old post, but there is a much simpler answer to this question and I wish someone had put it this way when I was looking for the answer myself as a beginner- I hope you're ok with reviving an old thread!
Its more readable using named functions and they are also capable of self-referencing as in the example below.
(function recursion(iteration){
if (iteration > 0) {
console.log(iteration);
recursion(--iteration);
} else {
console.log('done');
}
})(20);
console.log('recursion defined? ' + (typeof recursion === 'function'));
http://jsfiddle.net/Yq2WD/
This is nice when you want to have an immediately invoked function that references itself but does not add to the global namespace. It's still readable but not polluting. Have your cake and eat it to.
Hi, my name is Jason OR hi, my name is ???? you pick.
Well, just to be clear for the sake of my arguments, the following are all anonymous functions/function expressions in my book:
var x = function(){ alert('hi'); },
indexOfHandyMethods = {
hi: function(){ alert('hi'); },
high: function(){
buyPotatoChips();
playBobMarley();
}
};
someObject.someEventListenerHandlerAssigner( function(e){
if(e.doIt === true){ doStuff(e.someId); }
} );
(function namedButAnon(){ alert('name visible internally only'); })()
Pros:
It can reduce a bit of cruft, particularly in recursive functions (where you could (should actually since arguments.callee is deprecated) still use a named reference per the last example internally), and makes it clear the function only ever fires in this one place.
Code legibility win: in the example of the object literal with anon funcs assigned as methods, it would be silly to add more places to hunt and peck for logic in your code when the whole point of that object literal is to plop some related functionality in the same conveniently referenced spot. When declaring public methods in a constructor, however, I do tend to define labeled functions inline and then assign as references of this.sameFuncName. It lets me use the same methods internally without the 'this.' cruft and makes order of definition a non-concern when they call each other.
Useful for avoiding needless global namespace pollution - internal namespaces, however, shouldn't ever be that broadly filled or handled by multiple teams simultaneously so that argument seems a bit silly to me.
I agree with the inline callbacks when setting short event handlers. It's silly to have to hunt for a 1-5 line function, especially since with JS and function hoisting, the definitions could end up anywhere, not even within the same file. This could happen by accident without breaking anything and no, you don't always have control of that stuff. Events always result in a callback function being fired. There's no reason to add more links to the chain of names you need to scan through just to reverse engineer simple event-handlers in a large codebase and the stack trace concern can be addressed by abstracting event triggers themselves into methods that log useful info when debug mode is on and fire the triggers. I'm actually starting to build entire interfaces this way.
Useful when you WANT the order of function definition to matter. Sometimes you want to be certain a default function is what you think it is until a certain point in the code where it's okay to redefine it. Or you want breakage to be more obvious when dependencies get shuffled.
Cons:
Anon functions can't take advantage of function hoisting. This is a major difference. I tend to take heavy advantage of hoisting to define my own explicitly named funcs and object constructors towards the bottom and get to the object definition and main-loop type stuff right up at the top. I find it makes the code easier to read when you name your vars well and get a broad view of what's going on before ctrl-Fing for details only when they matter to you. Hoisting can also be a huge benefit in heavily event-driven interfaces where imposing a strict order of what's available when can bite you in the butt. Hoisting has its own caveats (like circular reference potential) but it is a very useful tool for organizing and making code legible when used right.
Legibility/Debug. Absolutely they get used way too heavily at times and it can make debug and code legibility a hassle. Codebases that rely heavily on JQ, for instance, can be a serious PITA to read and debug if you don't encapsulate the near-inevitable anon-heavy and massively overloaded args of the $ soup in a sensible way. JQuery's hover method for instance, is a classic example of over-use of anon funcs when you drop two anon funcs into it, since it's easy for a first-timer to assume it's a standard event listener assignment method rather than one method overloaded to assign handlers for one or two events. $(this).hover(onMouseOver, onMouseOut) is a lot more clear than two anon funcs.