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.
Related
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 an answer here:
Mystery of "this" in ReactJS
(1 answer)
Closed 3 years ago.
We all know that this will be undefined according to the given code of React. We have many solutions for this problem, like binding, arrow function, etc. I want to know the reason behind this behavior. Please, explain the reason for the behavior of this reference, instead of solutions.
class Foo extends Component {
clickHandler() {
console.log(this);
}
render() {
return <button onClick = {this.clickHandler}> Click Me </button>;
}
}
1)this.clickHandler=this.clickHandler.bind(this) is returns a new function,in which reference to 'this' will refer to the function this is the way of saving the current value of this which is scope during the call to the constructor so that it can be called later the function is called.
if our function dont require access to the state at your component then sure you dont need to bind this.
2)arrow functions automatically bind this, that’s why we don’t need to use .bind() method.
This question already has answers here:
When to use () after a callback function name? [duplicate]
(5 answers)
Closed 6 years ago.
Well I'm starting with jQuery and I'm wondering what is the difference between writing a function with or without parentheses inside a function.
For example if I have the following function:
function test(){
alert("pen pineapple apple pen");
}
This:
$(document).ready(test);
and this:
$(document).ready(test());
both show the same result: "pen pineapple apple pen"
Putting parentheses () at the end of a function causes that function to be invoked immediately, and use its return value in the expression.
This code $(document).ready(test); is using test as a callback function. It essentially says: when the document becomes ready, call the function that I'm providing you with (test).
This code $(document).ready(test()); is immediately invoking the function test, having it return a value, and then passing that value to the ready method. It's possible that test is returning a different function here, which in turn will act as the required callback function. It could also just be an error though, with someone inadvertently including the parentheses when they shouldn't have.
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?
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.