Can someone please clearly explain how to interpret the documentation for the collection.remove() method in backbone.js as seen here:
http://backbonejs.org/#Collection-remove
The docs show collection.remove(models, [options]), but I don't understand what this means for the actual usage of the function. What does options represent here? I'm confused because the callback also has an options argument as seen here:
http://backbonejs.org/#Events-catalog
The callback docs say:
"remove" (model, collection, options) — when a model is removed from a collection.
This means if I call collection.remove(model) somewhere in my code, my callbacks will be passed the collection, the model, and an options parameter. Is this options parameter the same one that can optionally be passed in on the remove call itself. If so, what are these options supposed to be used for?
If you would read the links that you posted, you will see this
"Generally speaking, when calling a function that emits an event (model.set, collection.add, and so on...), if you'd like to prevent the event from being triggered, you may pass {silent: true} as an option. Note that this is rarely, perhaps even never, a good idea. Passing through a specific flag in the options for your event callback to look at, and choose to ignore, will usually work out better."
So you can use the options to prevent the remove event from firing. You can also use it (as mentioned in the last sentence) to pass option values to the callback. So yes, anything you pass in as options to the remove function will be passed through as the options to the remove event.
Related
If I have multiple functions passed to a click event i.e.
#click="
inputHandler();
sendToken(computedUser.email);
responseMessage();
"
The function with an event parameter:
inputHandler(e) {
// code
}
Won't run. If I pass it on it's own:
#click="inputHandler"
It works fine.
Why is this and how can I get around it?
Internally Vue uses some RegExps to decide what form of event handler you're using.
If it seems to be the name of a method it will call it and pass the event.
If it seems to be inline code it'll just run it. In this case the event object is accessible as $event.
So:
#click="inputHandler($event)"
is roughly equivalent to:
#click="inputHandler"
Strictly speaking these aren't quite the equivalent, for example with component events that emit multiple arguments you'll only get the first one using $event like this.
See https://v2.vuejs.org/v2/guide/events.html#Methods-in-Inline-Handlers
For a deeper understanding see the Vue source code:
https://github.com/vuejs/vue/blob/0baa129d4cad44cf1847b0eaf07e95d4c71ab494/src/compiler/codegen/events.js#L96
Give your eyes a few minutes to adjust and it isn't too difficult to understand.
Personally I try to avoid anything more complicated than a single method call in the inline listener. Instead I'd suggest having something like #click="onSendClick" and let the method onSendClick worry about the details.
If I recall correctly, vue creates a wrapper function, if the passed value isn't a function. So
inputHandler();
sendToken(computedUser.email);
responseMessage();
actually get's turned into
function wrapper(){
inputHandler();
sendToken(computedUser.email);
responseMessage();
}
And as you can see the arguments passed to wrapper are lost.
The easiest way to fix this is probably to create a new method that accepts the event parameter and calls all of your function and use that one in the event handler.
Hi so I am just wanting to understand how javascript/react is handeling whats going on when I have a function triggered from an event . Im going to show three different code segements, 2 of which work, one of which didnt, and see if I am understanding them correctly. :)
setField(e){console.log("in setfield", e)}
<select multiple className="form-control" id="sel2" name="sellist2"
onChange={() => this.setField()}>
this doesnt work because it is passing a null value to a function that expects a value, in this case e. however, if the function was just submitting something already in state, that would be reasonable way to do it.
setField(e){console.log("in setfield", e)}
<select multiple className="form-control" id="sel2" name="sellist2"
onChange={(e) => this.setField(e)}>
this works because it is getting the event as e and then passes it to the function. How does it know to associate the event to e? does this functionality extent to other things when dealing with html?
setField(e){console.log("in setfield", e)}
<select multiple className="form-control" id="sel2" name="sellist2"
onChange={this.setField}>
This one I really dont understand why it works, but it does. I am assuming that it inherently know to pass the event as default to the function. again does this inbuilt logic occur anywhere else, maybe not neccessarily for events?
Thanks again. Im new to javascript so it is interesting to learn all the idosyncracies of javascript. sadly most of then learning is through frustration of debugging!
All functions in javascript can be passed an arbitrarily large number of parameters, even if the function doesn't do anything with them. For example, the following is legal, if useless:
function thingThatExpectsZeroParams() {
console.log('hello');
}
thingThatExpectsZeroParams(1, 2, 3, 4, [5], 'six');
Six pieces of data were passed into the function, but nothing happened with them because the function didn't do anything with them. To get access to the values passed in, you just need to pick a name that you're going to call it. This name is local to your function and can be anything you want.
function callback(event) {
console.log(event);
}
// I could also just have easily called it something else, and the code would work just as well
// function callback(quesoBurrito) {
// console.log(quesoBurrito);
// }
callback('hello');
So when you set up a listener like this:
onChange={() => this.setField()}
Your function will be called, passing in the event object. But since your function doesn't give the first parameter a name, you have no way to access it, and then when you call this.setField, you pass nothing in there either.
In contrast, with this listener:
onChange={(e) => this.setField(e)}
Your function will again be called, passing in the event object. This time, you named that argument 'e', and can then do with it what you will, in this case forwarding it along to setField.
And when you do this:
onChange={this.setField}
You cut out the middleman. this.setField will get called directly, passing in the event object. Presumably, setField is expecting this, and does something with the object it was passed.
It's not complicated. Whatever Javascript function you assign to onChange will get called at the appropriate time and passed one single argument generally known as e. You can give that argument any name you want in your function (or not declare it at all), but the first argument will be there.
So, when you do this:
onChange={() => this.setField()}
You are ignoring the argument passed to your event handler and then inside that event handler you're calling this.setField() which to no surprise does not pass any argument to setField(). In reality, what's actually happening is this:
onChange={(e) => this.setField()}
How does it know to associate the event to e? does this functionality extent to other things when dealing with html?
The DOM specification says that when an onChange event handler is called, it will be passed the e object as the first argument (whether you declare it or not, it's there as the first argument to the function). This is just how callbacks work in Javascript. The caller of the callback decides what arguments they are going to pass the callback. It's up to you when you declare your callback whether you declare the argument and use it or not. Regardless of how you declare it, the caller passes it.
For this section of code:
onChange={this.setField}
You are say that the function you want to be the onChange event handler is your setField() method. So, no surprise, when the DOM calls that method, it will pass the event object as the first argument just like in the above example.
This one I really dont understand why it works, but it does. I am assuming that it inherently know to pass the event as default to the function. again does this inbuilt logic occur anywhere else, maybe not neccessarily for events?
As described earlier the DOM specification is where this event handler is specified. In the definition of that, it describes what arguments will or won't be passed to whatever callback function you register as this event handler. This is true of all callbacks in Javascript. The caller decides what it will pass to the callback. This is not specific to this type of event handler. Callbacks are used lots of places in Javascript. Even something like array.filter() takes a callback and it's the caller of the callback (the implementation of the Array object in that case) that determines what arguments are passed to the callback.
In short, I'm doing this:
function myHandler(a,b,c,d){
doStuffWithMyParams(a,b,c,d);
}
Then somewhere else:
jqueryElem.click(myHandler.bind(a,b,c,d));
When I do, some of the parameters passed (a,b) are read correctly. But the third (c) is a JQuery event object. I've also tried binding the args as an array. Then, the first param becomes the event object.
Totally perplexed here. Thanks in advance for any direction on this.
With the code myHandler.bind(a,b,c,d), the argument a is the context that bind() uses
So my handler is actually seeing this
function myHandler(b,c,d,event){
So I have a feeling you want
jqueryElem.click(myHandler.bind(this, a,b,c,d));
Building on what epascarello wrote, this should do the trick:
jqueryElem.click(myHandler.bind(this,[a,b,c,d]));
Not having the array might cause b to be treated as an eventHandler. Notice I used an array, but it could also be an object, for instance.
I have searched prior SO posts here, here and here, and couldn't an answer that made sense to me. This should be a basic question, but I'm not understanding the posts I find. They don't seem to address using a this parameter.
I want to programatically add an input with an onchange event, such that the final result is this:
<input type="button" onchange="handleButtonOnChange(this)">ClickMe</input>
I am working on a project that is using an embedded IE6 browser inside a old Delphi application, so I have to have a solution that is IE6 compatible (yes, IE6 is horrible, but there are reasons I am stuck with it for now).
My initial attempt was this:
var DaySelect = document.createElement("select");
DaySelect.id = ParentID+"-day";
DaySelect.disabled = true;
MonthSelect.onchange="handleDayChange(this);" //<--- not correct
Parent.appendChild(DaySelect);
I then read that the .onchange should be assigned an object, not a string, and one should use this instead:
MonthSelect.onchange=handleDayChange; //<--- '(this)' removed
But it seem to me that this will result in this element (notice the missing this parameter)
<input type="button" onchange="handleButtonOnChange">ClickMe</input>
If I use the line below, instead, won't this make a closure, and the 'this' will refer to the event at the time the object is assigned to the .onchange property, instead of being the event at the time of the change event?
//Does the line below make a closure?
MonthSelect.onchange=handleDayChange(this); //<-- What does 'this' refer to?
I'm a relatively new web programmer, but long time Delphi programmer. Closures still make my head hurt. I appreciate any help in this.
Also, I read here about using addEventListener and the problems with older versions of IE, and the last post on the page provides a work around. But I don't understand how it works.
EDIT -- And what about passing other parameters? It seems that many event handlers will need to have parameters specific for the attached element. It seems that it is just not possible to add a listener with any parameters.
A simple closure if you are creating the elements in JS as you show:
var DaySelect = document.createElement("select");
DaySelect.id = ParentID+"-day";
DaySelect.disabled = true;
MonthSelect.onchange=function(){handleDayChange(DaySelect);};
Parent.appendChild(DaySelect);
Since the function is created inside the scope that you create the element in, the same variables will be available to it.
EDIT:
Additional parameters can be passed with this method, for example, the anonymous function we create and attach as the handler will still have the event object sent to it:
function(e){handleDayChange(DaySelect, e);};
In the event object you will have access to the event target, but in your example the event target and "this" are not the same element, so there would be no way for the handler to know about the DaySelect element.
jQuery makes a lot of event handling much simpler which is one of the reasons many people use it, it also normalizes it's methods between various browsers so you don't have to write multiple versions of the same code (in most cases)
Instructions:
make this code work without modifying snippet it in any way.
Use only plain old JavaScript and no third-party libraries.
Write new code that enables the code below to work properly.
Hint: Feel free to extend native objects... even though it's typically a bad practice.
// Start with an object, any object
var myObject = {};
// Register an event on your object using
// an `on` method
myObject.on('myEvent', function(data) {
// Log the data passed to the callback
console.log(data);
});
// Trigger the event using a `trigger` method.
// Include some data when you trigger the event.
myObject.trigger('myEvent', {
company: 'ABC Corp',
location: 'WTC Bangalore, IN',
website: 'http://abc.co'
});
// Register a different event
myObject.on('yourEvent', function() {
console.log('yourEvent fired');
});
// Trigger the new event
myObject.trigger('yourEvent');
// Trigger all existing events using a special
// "star" identifier.
myObject.trigger('*');
// Remove one event by name
myObject.off('myEvent');
// Since we've removed the event, this should
// do nothing
myObject.trigger('myEvent');
// Remove all existing events
myObject.off();
// Since we've removed all events, this should
// do nothing
myObject.trigger('*');
Everything else went well. I'm unable to get "arguments" while implementing myObject.trigger("*"); unable to read arguments object / parameters while implementing "*" and hence throw undefined.
My JSFiddle
Disclaimer
I obviously dont know what school you go to or anything, but please don't fool yourself trying to fool your teachers. With a few simple questions they'll know if you understand the material or not, and if you show up with a good answer but no knowledge to back it up, they will know what's up. I'm not accusing you of this, just a friendly word of advice of someone who has had good connections with his teachers after graduating last year ;)
So, how do we do this? Basically, you will have to add some functionality to the prototype of object, at least if you want this to affect all objects made afterwards. You can always create your own class and add the function to that prototype if you only want that class to have this functionality.
We need 3 functions added to the prototype, on, off and trigger of course. On top of that we add one extra property called events, initially an empty object.
You can look at the raw code for all these in the jsfiddle, I will only go through the structure and logic of the code here.
events will hold all the handlers (functions) associated with each event. When adding an event for the first time, we add a eventName property to the events object, the value for this property is initially an empty array.
on will find (or create) the array linked to eventName in events, and push the function into the array (note we do not call the function at this time, we simply store the reference to the function in the array).
off will iterate the array of eventName, and if it finds the same function (note the ===), remove it from the array.
trigger will iterate the array of eventName and call each function. Note that the function is called with the this keyword in the function set to the object, and with the same parameters as the trigger function was called (except eventName, the first parameter, which is filtered out). Yes that means you can pass as many parameters as you want to trigger(), and they will all be passed to each handler.
I won't go into detail what things like splice, slice, ===, arguments and apply do exactly, I'm sure you can find more and better information about that elsewhere on the world wide interwebs.
There's a lot more you can do for this, like making the events object invisible through some nice uses of scoping, but that wasn't part of the question so I didn't bother with that.
If you have any more questions after looking through this, feel free to ask. I also didn't test it extensively so if you find any bugs, let me know.
EDIT: I didn't read through the comments at first, but I now also added support for the '*' wildcard. Basically the functions now check for the wildcard and will iterate all eventNames on the event object when removing or triggering. You can also remove all functions for an event by not giving a function or by giving the same wildcard, but with an eventName.
EDIT2: had some bugs running the teacher's code, realized I forgot to check for hasOwnProperty while iterating. Look that one up, it's very important when working with prototypes!
I now put in the teacher's code in my jsfiddle, to show you that it works :)
jsfiddle with own code
jsfiddle with teacher code
EDIT3 - about the 'undefined' log.
The teacher's code calls .trigger 5 times, and you should see 4 console logs and as far as I can tell, they are all correct.Let me run through each trigger, and the subsequent console logs.
You add a handler to myEvent, which logs the first parameter
You trigger myEvent, with parameter => The parameter (the object), is
logged.
You add a handler to yourEvent, which logs a hardcoded
string.
You trigger yourEvent, no parameter => The hardcoded string is logged'
You trigger * with no parameter, all handlers run => undefined is logged, since no parameters were given, data in myEvent's handler is undefined. The hardcoded string is also logged
You remove the myEvent handler, trigger myEvent and confirm no functions are called
You remove all event handlers, trigger * and confirm no functions are called from any events.
I honestly don't know what you expected to happen on step 5, since you give no parameter, the data is assigned undefined, that's intended behaviour.
If you want to merge the data given in step 2 so it remains on the object, then instruct so in your handler. (for example, iterate all properties of data and add them to this, then log this). Right now you simply pass it data, it gets logged, and then thrown away. You can also add a parameter in step 5, and then all handlers will receive it (including the yourEvent handlers, but that one doesn't assign nor use it).
document.getElementById("myBtn").addEventListener("click", displayDate);