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I am using the modular design pattern for JS and I keep running into issues when using arguments bound functions. I have a particular function that I would like to bind to different events to keep from having to write the function for each bound event. The only difference in the function, or the argument, is the table that will be updated. The problem is that when I build a function with the arguments I need and pass those arguments to bound events, I get an undefined error, in the console, on load. Keep in mind, I want to stick with this design pattern for the security it offers.
Here is my JS:
var Users = (function(){
var $addRoleForm = $('#addUserRole');
var $rolesTableBody = $('#table-roles tbody');
$addRoleForm.submit(ajaxUpdate(event, $rolesTableBody));
function ajaxUpdate(event, tableName) {
event.preventDefault();
event.stopPropagation();
var url = this.action;
var data = $(this).serialize();
var $this = $(this);
$.ajax({
type: 'POST',
url: url,
dataType: 'json',
data: data,
success: function(data) {
if(data.st === 0){
$messageContainer.html('<p class="alert alert-danger">' + data.msg + '</p>');
setTimeout(function(){
$messageContainer.hide();
}, 7000);
} else {
$messageContainer.html('<p class="alert alert-success">' + data.msg + '</p>');
tableName.fadeOut().html('').html(data.build).fadeIn();
$this.find('input').val('');
setTimeout(function(){
$messageContainer.hide();
}, 7000);
}
},
error: function(xhr, status, error){
console.log(xhr.responseText);
}
});
}
})();
Here is the error I get in the console, on load:
Uncaught TypeError: Cannot read property 'preventDefault' of undefined
I have tried to bind the event like this: $addRoleForm.on('submit', ajaxUpdate(event, $rolesTableBody)); and receive the same results.
Any ideas how to fix this?
You're seeing that issue, because the way you have it written now, ajaxUpdateexecutes, returns undefined and THEN passes undefined to the event listener, so you're basically doing this: $addRoleForm.submit(undefined).
2 Choices here:
1) You can wrap it in an anonymous function:
$addRoleForm.submit(function(event) {
//pass the value of "this" along using call
ajaxUpdate.call(this, event, someValue);
});
$someOtherForm.submit(function(event) {
//pass the value of "this" along using call
ajaxUpdate.call(this, event, someOtherValue);
});
2) You can set the first argument in-advance using bind:
$addRoleForm.submit(ajaxUpdate.bind($addRoleForm, someValue));
$someOtherForm.submit(ajaxUpdate.bind($someOtherForm, someOtherValue));
Using this way, you're binding the value of this to be $addRoleForm, setting the first argument to always be someValue, so it's the same as:
ajaxUpdate(someValue, event) {
//value of "this" will be $addRoleForm;
}
To pass the event, and the custom argument, you should be using an anonymous function call
$addRoleForm.submit(function(event) {
ajaxUpdate(event, $rolesTableBody));
});
This is by far the easiest and most readable way to do this.
What you're doing right now equates to this
var $addRoleForm = $('#addUserRole');
var $rolesTableBody = $('#table-roles tbody');
var resultFromCallingFunction = ajaxUpdate(event, $rolesTableBody); // undefined
$addRoleForm.submit(resultFromCallingFunction);
Where you're calling the ajaxUpdate function, as that's what the parentheses do, and pass the returned result back to the submit callback, which in your case is undefined, the default value a function returns when nothing else is specified.
You could reference the function, like this
$addRoleForm.submit(ajaxUpdate);
but then you can't pass the second argument
The question refers to the Revealing Module pattern. Benefit of using this design is readability. Going with the anon function may work, but defeats the overall purpose of the module pattern itself.
A good way to structure your module to help maintain your scope is to setup helper functions first, then call a return at the end.
Example use case with events:
var User = function() {
// local VARS available to User
var addRoleForm = document.querySelector('#addUserRole');
var rolesTableBody = document.querySelector('#table-roles tbody');
// Helper function 1
function ajaxUpdate(tableName) {
...
}
// Helper function 2
function someFunc() {
...
}
function bindEvents() {
addRoleForm.addEventListener('submit', ajaxUpdate, false);
addRoleForm.addEventListener('click', someFunc, false);
}
function init() {
bindEvents();
}
return {
runMe:init
}
}().runMe();
Helps to "modularize" your workflow. You are also writing your revealing pattern as an IIFE. This can cause debugging headaches in the future. Editing the IIFE to instead invoke via the return is easier to maintain and for other devs to work with and learn initially. Also, it allows you to extend outside of your IFFE into another Module, example:
var Clothes = function() {
function anotherFunc() {
...
}
init() {
User.runMe();
anotherFunc();
}
return {
addClothes: init
}
}().addClothes();
I hope this helps to give you a better understanding of how/when/why to use the JS revealing pattern. Quick note: You can make your modules into IIFE, that's not a problem. You just limit the context of the scope you can work with. Another way of doing things would be to wrap the var User and var Clothes into a main module, and then make that an IIFE. This helps in preventing polluting your global namespace.
Example with what I wrote above:
// MAIN APPLICATION
var GettinDressed = (function() {
// MODULE ONE
///////////////////////////
Var User = function() {
// local VARS available to User
var addRoleForm = document.querySelector('#addUserRole');
var rolesTableBody = document.querySelector('#table-roles tbody');
// Helper function 1
function ajaxUpdate(tableName) {
...
}
// Helper function 2
function someFunc() {
...
}
function bindEvents() {
addRoleForm.addEventListener('submit', ajaxUpdate, false);
addRoleForm.addEventListener('click', someFunc, false);
}
function init() {
bindEvents();
}
return {
runMe:init,
style: someFunc
}
}();
// MODULE TWO
//////////////////////////
var Clothes = function() {
function anotherFunc() {
...
}
init() {
User.style();
anotherFunc();
}
return {
dressUp: init
}
}();
// Define order of instantiation
User.runMe();
Clothes.dressUp();
}());
Here is a simplified snippet from some code I wrote for managing tablet gestures on canvas elements
first a function that accepts an element and a dictionary of callbacks and register the events plus adding other features like 'hold' gestures:
function registerStageGestures(stage, callbacks, recieverArg) {
stage.inhold = false;
stage.timer = null;
var touchduration = 1000;
var reciever = recieverArg || window;
stage.onLongTouch = function(e) {
if (stage.timer) clearTimeout(stage.timer);
stage.inhold = true;
if (callbacks.touchholdstart) callbacks.touchholdstart.call(reciever, e);
};
stage.getContent().addEventListener('touchstart', function(e) {
e.preventDefault();
calcTouchEventData(e);
stage.timer = setTimeout(function() {
stage.onLongTouch(e);
}, touchduration);
if (callbacks.touchstart) callbacks.touchholdstart.call(reciever, e);
});
stage.getContent().addEventListener('touchmove', function(e) {
e.preventDefault();
if (stage.timer) clearTimeout(stage.timer);
if (stage.inhold) {
if (callbacks.touchholdmove) callbacks.touchholdmove.call(reciever, e);
} else {
if (callbacks.touchmove) callbacks.touchmove.call(reciever, e);
}
});
stage.getContent().addEventListener('touchend', function(e) {
e.preventDefault();
if (stage.timer) clearTimeout(stage.timer);
if (stage.inhold) {
if (callbacks.touchholdend) callbacks.touchholdend.call(reciever, e);
} else {
if (callbacks.touchend) callbacks.touchend.call(reciever, e);
}
stage.inhold = false;
});
}
later I call registerStageGestures on a few elements (represented by 'View' objects) in the same page. Something like:
function View() {
var self=this;
..
function InitView() {
...
registerStageGestures(kineticStage, {
touchstart: function(e) {
// do something
},
touchmove: function(e) {
// do something
},
touchendunction(e) {
// do something
},
touchholdstart: function(e) {
// do something
},
touchholdmove: function(e) {
// do something
},
touchholdend: function(e) {
// do something
},
}, self);
Everything works fine, however I'm left wondering about two things in the implementation of registerStageGestures:
First, is it necessary to make inhold, timer and onLongTouch members of the stage ? or will closures make everything works well if they are local vars in registerStageGestures ?
Second, is it necessary to call the callbacks with '.call(receiver,' syntax ? I'm doing this to make sure the callback code will run in the context of the View but I'm not sure if it's needed ?
any input is much appreciated
Thanks!
First, is it necessary to make inhold, timer and onLongTouch members
of the stage ? or will closures make everything works well if they are
local vars in registerStageGestures ?
As far as registerStageGestures() is concerned, var inhold, var timer and function onLongTouch(e) {...}. would suffice. The mechanism by which an inner function has automatic access to its outer function's members is known as "closure". You would only need to set stage.inhold, stage.timer and stage.onLongTouch if some other piece of code needs access to these settings as properties of stage.
Second, is it necessary to call the callbacks with '.call(receiver,'
syntax ? I'm doing this to make sure the callback code will run in the
context of the View but I'm not sure if it's needed ?
Possibly, depending on how those callbacks are written. .call() and .apply() are sometimes used when calling functions that use this internally. In both cases, the first parameter passed defines the object to be interpreted as this. Thus, javascript gives you the means of defining general purpose methods with no a priori assumption about the object to which those methods will apply when called. Similarly, you can call a method of an object in such a way that it acts on another object.
EDIT:
For completeness, please note that even in the absence of this in a function, .apply() can be very useful as it allows multiple parameters to be specified as elements of a single array, eg the ubiquitous jQuery.when.apply(null, arrayOfPromises)...
There are some simple answers, here.
First, closure:
Closure basically says that whatever is defined inside of a function, has access to the rest of that function's contents.
And all of those contents are guaranteed to stay alive (out of the trash), until there are no more objects left, which ere created inside.
A simple test:
var testClosure = function () {
var name = "Bob",
recallName = function () { return name; };
return { getName : recallName };
};
var test = testClosure();
console.log(test.getName()); // Bob
So anything that was created inside can be accessed by any function which was also created inside (or created inside of a function created in a function[, ...], inside).
var closure_2x = function () {
var name = "Bob",
innerScope = function () {
console.log(name);
return function () {
console.log("Still " + name);
}
};
return innerScope;
};
var inner_func = closure_2x();
var even_deeper = inner_func(); // "Bob"
even_deeper(); // "Still Bob"
This applies not only to variables/objects/functions created inside, but also to function arguments passed inside.
The arguments have no access to the inner-workings(unless passed to methods/callbacks), but the inner-workings will remember the arguments.
So as long as your functions are being created in the same scope as your values (or a child-scope), there's access.
.call is trickier.
You know what it does (replaces this inside of the function with the object you pass it)...
...but why and when, in this case are harder.
var Person = function (name, age) {
this.age = age;
this.getAge = function () {
return this.age;
};
};
var bob = new Person("Bob", 32);
This looks pretty normal.
Honestly, this could look a lot like Java or C# with a couple of tweaks.
bob.getAge(); // 32
Works like Java or C#, too.
doSomething.then(bob.getAge);
? Buh ?
We've now passed Bob's method into a function, as a function, all by itself.
var doug = { age : 28 };
doug.getAge = bob.getAge;
Now we've given doug a reference to directly use bobs methid -- not a copy, but a pointer to the actual method.
doug.getAge(); // 28
Well, that's odd.
What about what came out of passing it in as a callback?
var test = bob.getAge;
test(); // undefined
The reason for this, is, as you said, about context...
But the specific reason is because this inside of a function in JS isn't pre-compiled, or stored...
this is worked out on the fly, every time the function is called.
If you call
obj.method();
this === obj;
If you call
a.b.c.d();
this === a.b.c;
If you call
var test = bob.getAge;
test();
...?
this is equal to window.
In "strict mode" this doesn't happen (you get errors really quickly).
test.call(bob); //32
Balance restored!
Mostly...
There are still a few catches.
var outerScope = function () {
console.log(this.age);
var inner = function () {
console.log("Still " + this.age);
};
inner();
};
outerScope.call(bob);
// "32"
// "Still undefined"
This makes sense, when you think about it...
We know that if a function figures out this at the moment it's called -- scope has nothing to do with it...
...and we didn't add inner to an object...
this.inner = inner;
this.inner();
would have worked just fine (but now you just messed with an external object)...
So inner saw this as window.
The solution would either be to use .call, or .apply, or to use function-scoping and/or closure
var person = this,
inner = function () { console.log(person.age); };
The rabbit hole goes deeper, but my phone is dying...
I'm using Class.js for creating classes.
I'm not getting the right context inside a method when invocked from a call back function
My code is
WordCloud = MyClass.extend({
init: function(data) {
var me = this;
(......).on("onComplete", this.draw);
},
show: function(word) {
alert(word)
},
draw : function(words){
console.debug(this); // prints element that triggred `onComplete` action
console.debug(words); // "Hi"
console.debug(me); // me is not defined
me.show(words) // Need to call this method
}
});
Problem is draw method is fired when an action is completed, but inside draw method this is not the actual class instance, but the element that triggred the callback action.
I can't pass exta arguments while calling this.draw as it is a call back function and onComplete has only one parameter.
How can I call the show method from draw?
If you do not have to support Internet Explorer 8 or lower, you can use bind():
init: function(data) {
var me = this;
(......).on("onComplete", this.draw.bind(this));
}
Otherwise, if you're already using jQuery, you can leverage $.proxy(), which works the same way:
init: function(data) {
var me = this;
(......).on("onComplete", $.proxy(this.draw, this));
}
I use a helper function for these cases.
function hitch(obj, func) {
return function() {
return obj[func].apply(obj, arguments || [])
};
}
To call it you would use hitch(this, 'draw'); instead of this.draw.
Or to make it even simpler you could add a simplified version to your base class
function hitch(func) {
var that = this;
return function() {
return that[func].apply(that, arguments || [])
};
}
And just call this.hitch('draw');.
I have two Backbone collections. I want to bind to the reset event one one. When that event is fired, I want to call fetch on the second collection, like so:
App.collections.movies.bind("reset", App.collections.theaters.fetch);
The second fetch never fires though. However, if I pass an anonymous function that calls theaters.fetch, it works no problem:
App.collections.movies.bind("reset", function () { App.collections.theaters.fetch(); });
Any idea why this might be the case?
Heres my full code. I'm not showing any of the models or collections, because it's a lot of code, but let me know if you think that might be the source of the problem:
var App = {
init: function () {
App.collections.theaters = new App.Theaters();
App.collections.movies = new App.Movies();
App.events.bind();
App.events.fetch();
},
events: {
bind: function () {
App.collections.theaters.bind("reset", App.theaterManager.assign);
App.collections.movies.bind("reset", function () { App.collections.theaters.fetch(); });
},
fetch: function () {
App.collections.movies.fetch();
}
},
collections: {},
views: {},
theaterManager: {
// Provide each model that requires theaters with the right data
assign: function () {
// Get all theaters associated with each theater
App.theaterManager.addToCollection("theaters");
// Get all theaters associated with each movie
App.theaterManager.addToCollection("movies");
},
// Add theaters to a collection
addToCollection: function (collection) {
App.collections[collection].each(function (item) {
item.theaters = App.theaterManager.getTheaters(item.get(("theaters")));
});
},
// Returns a collection of Theaters models based on a list of ids
getTheaters: function () {
var args;
if (!arguments) {
return [];
}
if (_.isArray(arguments[0])) {
args = arguments[0];
} else {
args = Array.prototype.slice.call(arguments);
}
return new App.Theaters(_.map(args, function (id) {
return App.collections.theaters.get(id);
}));
}
}
};
$(function () {
App.init();
});
This all has to do with function context. It is a common confusion with the way functions are called in Javascript.
In your first way, you are handing a function to be called, but there is no context defined. This means that whoever calls it will become "this". It is likely that the equivalent will be of calling App.collections.movies.fetch() which is not what you want. At least, I am guessing that is what the context will be. It is difficult to know for sure... it might be jQuery, it might be Backbone.sync. The only way to tell is by putting a breakpoint in the Backbone.collections.fetch function and print out the this variable. Whatever the case, it won't be what you want it to be.
In the second case, you hand it a function again but internally, you specify the context in which the function is called. In this case, fetch gets called with App.collections.theaters as the context.
... was that clear?
The main reason why I want it is that I want to extend my initialize function.
Something like this:
// main.js
window.onload = init();
function init(){
doSomething();
}
// extend.js
function extends init(){
doSomethingHereToo();
}
So I want to extend a function like I extend a class in PHP.
And I would like to extend it from other files too, so for example I have the original init function in main.js and the extended function in extended.js.
With a wider view of what you're actually trying to do and the context in which you're doing it, I'm sure we could give you a better answer than the literal answer to your question.
But here's a literal answer:
If you're assigning these functions to some property somewhere, you can wrap the original function and put your replacement on the property instead:
// Original code in main.js
var theProperty = init;
function init(){
doSomething();
}
// Extending it by replacing and wrapping, in extended.js
theProperty = (function(old) {
function extendsInit() {
old();
doSomething();
}
return extendsInit;
})(theProperty);
If your functions aren't already on an object, you'd probably want to put them there to facilitate the above. For instance:
// In main.js
var MyLibrary = {
init: function init() {
}
};
// In extended.js
(function() {
var oldInit = MyLibrary.init;
MyLibrary.init = extendedInit;
function extendedInit() {
oldInit.call(MyLibrary); // Use #call in case `init` uses `this`
doSomething();
}
})();
But there are better ways to do that. Like for instance, providing a means of registering init functions.
// In main.js
var MyLibrary = (function() {
var initFunctions = [];
return {
init: function init() {
var fns = initFunctions;
initFunctions = undefined;
for (var index = 0; index < fns.length; ++index) {
try { fns[index](); } catch (e) { }
}
},
addInitFunction: function addInitFunction(fn) {
if (initFunctions) {
// Init hasn't run yet, remember it
initFunctions.push(fn);
} else {
// `init` has already run, call it almost immediately
// but *asynchronously* (so the caller never sees the
// call synchronously)
setTimeout(fn, 0);
}
}
};
})();
Here in 2020 (or really any time after ~2016), that can be written a bit more compactly:
// In main.js
const MyLibrary = (() => {
let initFunctions = [];
return {
init() {
const fns = initFunctions;
initFunctions = undefined;
for (const fn of fns) {
try { fn(); } catch (e) { }
}
},
addInitFunction(fn) {
if (initFunctions) {
// Init hasn't run yet, remember it
initFunctions.push(fn);
} else {
// `init` has already run, call it almost immediately
// but *asynchronously* (so the caller never sees the
// call synchronously)
setTimeout(fn, 0);
// Or: `Promise.resolve().then(() => fn());`
// (Not `.then(fn)` just to avoid passing it an argument)
}
}
};
})();
There are several ways to go about this, it depends what your purpose is, if you just want to execute the function as well and in the same context, you can use .apply():
function init(){
doSomething();
}
function myFunc(){
init.apply(this, arguments);
doSomethingHereToo();
}
If you want to replace it with a newer init, it'd look like this:
function init(){
doSomething();
}
//anytime later
var old_init = init;
init = function() {
old_init.apply(this, arguments);
doSomethingHereToo();
};
The other methods are great but they don't preserve any prototype functions attached to init. To get around that you can do the following (inspired by the post from Nick Craver).
(function () {
var old_prototype = init.prototype;
var old_init = init;
init = function () {
old_init.apply(this, arguments);
// Do something extra
};
init.prototype = old_prototype;
}) ();
Another option could be:
var initial = function() {
console.log( 'initial function!' );
}
var iWantToExecuteThisOneToo = function () {
console.log( 'the other function that i wanted to execute!' );
}
function extendFunction( oldOne, newOne ) {
return (function() {
oldOne();
newOne();
})();
}
var extendedFunction = extendFunction( initial, iWantToExecuteThisOneToo );
2017+ solution
The idea of function extensions comes from functional paradigm, which is natively supported since ES6:
function init(){
doSomething();
}
// extend.js
init = (f => u => { f(u)
doSomethingHereToo();
})(init);
init();
As per #TJCrowder's concern about stack dump, the browsers handle the situation much better today. If you save this code into test.html and run it, you get
test.html:3 Uncaught ReferenceError: doSomething is not defined
at init (test.html:3)
at test.html:8
at test.html:12
Line 12: the init call, Line 8: the init extension, Line 3: the undefined doSomething() call.
Note: Much respect to veteran T.J. Crowder, who kindly answered my question many years ago, when I was a newbie. After the years, I still remember the respectfull attitude and I try to follow the good example.
This is very simple and straight forward. Look at the code. Try to grasp the basic concept behind javascript extension.
First let us extend javascript function.
function Base(props) {
const _props = props
this.getProps = () => _props
// We can make method private by not binding it to this object.
// Hence it is not exposed when we return this.
const privateMethod = () => "do internal stuff"
return this
}
You can extend this function by creating child function in following way
function Child(props) {
const parent = Base(props)
this.getMessage = () => `Message is ${parent.getProps()}`;
// You can remove the line below to extend as in private inheritance,
// not exposing parent function properties and method.
this.prototype = parent
return this
}
Now you can use Child function as follows,
let childObject = Child("Secret Message")
console.log(childObject.getMessage()) // logs "Message is Secret Message"
console.log(childObject.getProps()) // logs "Secret Message"
We can also create Javascript Function by extending Javascript classes, like this.
class BaseClass {
constructor(props) {
this.props = props
// You can remove the line below to make getProps method private.
// As it will not be binded to this, but let it be
this.getProps = this.getProps.bind(this)
}
getProps() {
return this.props
}
}
Let us extend this class with Child function like this,
function Child(props) {
let parent = new BaseClass(props)
const getMessage = () => `Message is ${parent.getProps()}`;
return { ...parent, getMessage} // I have used spread operator.
}
Again you can use Child function as follows to get similar result,
let childObject = Child("Secret Message")
console.log(childObject.getMessage()) // logs "Message is Secret Message"
console.log(childObject.getProps()) // logs "Secret Message"
Javascript is very easy language. We can do almost anything. Happy JavaScripting... Hope I was able to give you an idea to use in your case.
as I understand it, you are trying to fetch the applications connected to the user account. You can do this by making a request on the API, I don't know if discord.js covers this part of the API
endpoint: https://discord.com/api/users/#me/connections
Request type: GET Header:
Authorization: "Beareryou token"
response: [
{...}
]
Use extendFunction.js
init = extendFunction(init, function(args) {
doSomethingHereToo();
});
But in your specific case, it's easier to extend the global onload function:
extendFunction('onload', function(args) {
doSomethingHereToo();
});
I actually really like your question, it's making me think about different use cases.
For javascript events, you really want to add and remove handlers - but for extendFunction, how could you later remove functionality? I could easily add a .revert method to extended functions, so init = init.revert() would return the original function. Obviously this could lead to some pretty bad code, but perhaps it lets you get something done without touching a foreign part of the codebase.