This question already has answers here:
How to access the correct `this` inside a callback
(13 answers)
Closed 6 years ago.
I have a listener like this one :
this.frame.on('touchend', self.findClosestSlide);
I need to have it like this, instead of inside an anonymous function in order to be able to unbind it later, using the function name.
My problem is that, once I am in the findClosestSlide function, my this object logically becomes this.frame.
How can I access the original this, or how can I have control on the context I'm sending to my callback function? (without using an anonymous function)
You can use Function.bind() or $.proxy() to pass a custom context to a callback like
Cross Browser, use $.proxy()
this.frame.on('touchend', $.proxy(self.findClosestSlide, self));
or IE9+, use function.bind()
this.frame.on('touchend', self.findClosestSlide.bind(self));
Why? because by default this inside the event handler will refer to the element that was targeted by the event
You can store this inside another variable such as that or $this than using it inside your findClosestSlide function.
Related
This question already has answers here:
Can you rebind a rebound function using `bind`
(3 answers)
How to override 'this' parameter of a bound function in javascript
(3 answers)
Javascript function bind override (how to bind it to another object)
(2 answers)
Closed 4 months ago.
I have a question about Function.prototype.bind.
function f() {
return this;
}
const g=f.bind({foo:'foo'});
const h=g.bind({bar:'bar'});
console.log(h()); // expected {bar:'bar'} but get {foo:'foo'}
I was expecting the above snippet to produce either {foo,bar} or {bar} but instead I get {foo}.
Can you explain what is going on?
Using .bind() to create a function that "fixes" the value of this in the called function results in a function where this cannot be overridden.
What you get back from .bind() is a new function that will pass along arguments to the original target function, with a guarantee that in that called function the value of this will be what you ask for. Re-binding that function therefore does no good: you do get back a new function, but it calls a function (the first bound function) that explicitly ignores its own this value.
This question already has answers here:
React Function Calling Methods
(3 answers)
Closed 8 months ago.
I have noticed that (at least in React) there are different ways to call a function. I'd say:
onClick={myFunction}
onClick={myFunction()}
onClick={()=>myFunction}
onClick={()=>myFunction()} /*Not sure if I've seen this type*/
Being the case all are correct, what are the difference between them?
onClick={myFunction}
Assigns myFunction to onClick.
onClick={myFunction()}
Calls myFunction immediately and assigns the return value (which needs to be another function) to onClick.
This is often an error caused by people not understanding how () works and wanting the previous syntax.
onClick={()=>myFunction}
Creates a new function and assigns it to onClick.
The new function mentions myFunction but doesn't do anything with it. It is a noop.
It is always a mistake.
onClick={()=>myFunction()}
Creates a new function, which calls myFunction, and assigns it to onClick.
This is usually a waste of resources. It is useful only if myFunction would do something with the event object that onClick passes to the event handler when called and you want to prevent that.
If you were to pass arguments to myFunction (which this example does not) then it would be more generally useful.
This question already has answers here:
When do I use parentheses and when do I not?
(5 answers)
Closed 6 years ago.
I'm having questions about when to call a function inside a react component. Sometimes my code breaks when I don't add the brackets to a function call, but not always. Is there some sort of rule i'm missing here?
Doesn't work
// Callback of parent component
<Link onClick={this.props.OnNavigate}>
A link
</Link>
Does work
// Callback of parent component
<Link onClick={this.props.OnNavigate()}>
A link
</Link>
// Callback for function of component
<li onClick={this.toggleDepartments}>other example</li>
foo() is calling the function referenced by foo. foo itself is just a reference to a function, it doesn't call the function.
So, you need to use parenthesis if you want to call the function right here and now.
You have to omit the parentheses if you want to pass the function to other code so it can call the function. That would be the case with event handlers. this.props.OnNavigation should be called when the click event happens (which is some time in the future), not when the component is rendered.
It's good practice to call the function with parens, because when you create a separate js file and link them together with the script tag, you know for certain that it's calling that specific function.
This question already has answers here:
What is the use of the JavaScript 'bind' method?
(23 answers)
Closed 6 years ago.
In some part of onInit function of a controller in a SAPUI5 application there is an auto generated code like this:
this.getView().addEventDelegate({
onBeforeFirstShow: function() {
// Some codes
}.bind(this)
});
Now my question is what does .bind(this) mean? What does it do? Is it a pure JavaScript code or it is related to SAPUI5?
Yes, it's pure JavaScript code, you can learn more about what bind is and does here
The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
In this case what it does is basically binding the current this to that function, so when onBeforeFirstShow is called, the this inside that function will be the one passed to the bind function.
You may also want to look at the new arrow function syntax in ES6, it auto binds the current this so bind(this) is not necessary.
It binds the listener of the function to the current class, then when you use this pointer inside of the onBeforeFirstShow function, the this pointer refer to encapsulated class and you can access to its members.
The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
Syntax
fun.bind(thisArg[, arg1[, arg2[, ...]]])
Reference to Mozilla Developer Network
This question already has answers here:
What is a callback function?
(22 answers)
Closed 7 years ago.
$("p").click(function(){
// action goes here!!
});
In the above jquery code why we pass a function() to the event?
It's called a callback or an anonymous function that you want executed when the event happens. You'll notice it's not just jQuery, but almost all Javascript frameworks will expect one. It's usually whenever you are binding to events, executing functions that might take a while to return, or functions that execute other functions.
But you can also provide it a function name if you want.
function clickedMe(){
alert("Something clicked me");
}
$("p").click(clickedMe)
Because the click method is defined this way (the method parameter ).
See:
click
You have to understand its not a default JavaScript method, this method is defined in the JQuery so you have to call it as it is defined in the method.
Behind the scene actually when JQuery registering any event with DOM its only says to do one thing which is calling your provide method.
The simplest answer: according to documentation.
https://api.jquery.com/click/
When a user clicks a p element, jQuery is supposed to fire an event and execute something. How is it supposed to pass actions, if not using functions?