onClick={someFunction} VS onClick={()=>someFunction}

The difference between onClick={someFunction} and onClick={() => someFunction} in React (or JavaScript in general) lies in how and when they handle the execution of the someFunction when the click event occurs :

onClick={someFunction}

• Direct Reference: This syntax directly references the function someFunction.
• When Clicked: The function someFunction will be called when the onClick event is triggered (e.g., when the element is clicked).
• No Extra Function Call: No additional function is created; React simply uses the function reference you provided.
• Example: If someFunction is defined as function someFunction() { console.log('clicked'); }, then onClick={someFunction} will log 'clicked' when the element is clicked.

onClick={() => someFunction()}

• Arrow Function: This syntax uses an arrow function to call someFunction.
• Creates a New Function: Each time the component renders, a new function is created. The arrow function wraps the call to someFunction.
• Immediate Invocation: Within the arrow function, someFunction is called immediately when the onClick event is triggered.
• Use Case: Useful when you need to pass arguments to the function or need to do additional work before calling someFunction.
• Example: If you want to pass an argument to someFunction, you can use onClick={() => someFunction('argument')}. This will call someFunction with 'argument' when the element is clicked.

When to Use Each

• Direct Reference (onClick={someFunction}):
  • Use this when you want to avoid creating an extra function on each render, which can be more efficient.
  • Preferable for simple event handlers where no additional arguments or operations are needed.
• Arrow Function (onClick={() => someFunction()}):
  • Use this when you need to pass arguments to the function or perform additional operations before calling the function.
  • Useful for inline operations or when dealing with closures.

Practical Examples

Direct Reference

function handleClick() {
  console.log('Button clicked');
}

<button onClick={handleClick}>Click Me</button>

• handleClick will be called directly when the button is clicked.

Arrow Function

function handleClick(message) {
  console.log(message);
}

<button onClick={() => handleClick('Button clicked')}>Click Me</button>

• The arrow function calls handleClick with the argument 'Button clicked' when the button is clicked.

Understanding these differences helps in optimizing performance and achieving the desired behavior in React components.

Comments

[Eckehard]

I don´t think this is actually correct.

A function is just just block of code between curly brackets, regardless if its defined with the function keyword or as an arrow function:

function f1(a){  return a*2 }  // function definition
const f2 = (a) => { return a*2 } // arrow function
const f3 = a => a*2 // short form of arrow function

so, this is a bit misleading / wrong:

Each time the component renders, a new function is created.

what varies is the execution context, not the function itself. It may sound a bit petty, but may lead to serious errors. It is important to get these differences exactly, as Javascript has some strange behavoir in this point (See here)[dev.to/brythewiseguy/context-in-ja...].

If you are using only pure functions, this should not matter too much unless you use the "this" keyword (which in my eyes makes the function impure)

So, calling one of this functions is pretty much the same, and might not make any measurable difference in a well optimizes Javascript compiler:

function f1(a){  return a*2 }  // function definition
const f2 = f1 // function reference
const f3 = a => f1(a) // function call inside an arrow function
const function f4(a){ return f1(a)}

It is more important to know, WHERE a function is defined and what it´s actual context is.

[Huy Phan]

so, this is a bit misleading / wrong:

Each time the component renders, a new function is created.

In the context of this article (React using JSX syntax), what the author said is correct. But to be more clear: Each time the component renders, a new anonymous function is created to pass to the onClick props.
Actually, using arrow function syntax here is not good because of the above reason, every time the parent renders, it passes a new prop value to the child, which leads the child to do the unnecessary rerender.

[Eckehard]

Maybe you can add some more details/references. Things are complicated here so its possible I´m wrong. I think it´s important if and when a new function object is created. See here

[Hasan Zohdy]

If the function receives a parameter like a we can use the same concept but with HOF (Higher Order Function) to be set like this:

const double = (value: number) => () => {
   // do whatever needed to be done
};

And the usage will be something like:

<div onClick={double(2)} />

[Mithu Ahammad]

it's Easy...

[Milan Vadhel]

Currying function

[Mario Francisco Randrianandrasana]

I think the article said about the component on each render is correct :

Each time the component renders, a new function is created.

function someFunction( ) { console.log('message') };
//here, it doesn't create a new function on click event, just call it
onClick={ someFunction } 
//here, it creates an anonymous function then call someFunction  
onClick={() => someFunction() } 

[itric]

i can't say, i fully understand you. i'm not an expert. i find it useful to share this info. Sorry for any misleading information, i'll be careful from next time. Thank you for correction

[Eckehard]

Your welcome!

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[Julian Gaston]

Nice article. Good stuff!!

[Eckehard]

I did some research to get more clarity on this topic. See this page for more details.

Javascript functions are just objects, so they can be extended. We can add a new property to see what´s going on. Here we add a counter to a function. Each time, the function is called, the counter is increased:

// function sayHi(){
const sayHi = () =>  {
  console.log("Hi");
  sayHi.counter++;
}
sayHi.counter = 0; // initial value

sayHi(); // Hi
sayHi(); // Hi

console.log( `Called ${sayHi.counter} times` ); // Called 2 times

So, we can clearly see, the function only exists once and is called twice. This works with regular functions and with arrow functions in the same way.

Things are a bit different if we create the complete function body inside the an arrow function like this:

run2 = () => {
  let f = function (){
    console.log("Hi2")
    f.counter ++
  }
  f.counter = 0
  return f
}

let x 
x = run2(); x()
x = run2(); x()
console.log( `Called ${x.counter} times` ); // Called 1 times

Here, each time run2 is called, we create a new function. But this happens only, if we create the whole function body inside the arrow function. It should not happen if we create the function outside and just use a reference.

[Syed Muhammad Ali Raza]

👍

[Raghuwar Jangir]

great content 👍for beginners it's good practice.

[Douglas Pujol]

Excellent article

[keshav jha]

There is alternate way in which you can use something like data-{xyz} attribute in element and onclick or on any event, you can get that attribute value using e.target.getAttribute("data-{xyz}").

function handleClick(e) {
const message = e.target.getAttribute("data-message")
console.log(message);
}

<button data-message="Button clicked" onClick={handleClick}>Click Me</button>

[Dillon Headley]

If you are using data attributes you should use e.target.dataset to read them. Like e.target.dataset.message

[keshav jha]

Yup, performance difference is negligible but .getAttribute is faster than dataset field some extent. That is not an excuse, the thing is i got used to .getAttribute and primarily i want to fetch only couple of data attributes in most of the scenario so i'm using .getAttribute mostly.

[Chavan-Soham]

So output will be "Button clicked" because you have passed this value to data-message attribute right ?

[GihanRangana]

This will be the cleanest way to pass arguments to the function

function handleClick(message) {
  console.log(message);
}

<button onClick={handleClick.bind(null,"Button Clicked")}>Click Me</button>

[Sateek Roy]

How? The .bind creates a new function every time the component is re-rendered. If we want to pass an argument to this function then the only way to prevent a new function from being created on a render is to create a memoized reference to that function using the useCallback hook.

const testFn = () => { console.log('Hello'); }
console.log(testFn.bind(null) === testFn.bind(null)); // false

Try this code in the browser console and you'll get false as the output which proves that bind will also cause a re-render.

[Edson Jr.]

Have you actually read what you wrote?
Cleaning up your sentences a bit I got:

onClick={someFunction}
The function someFunction will be called when the onClick event is triggered

onClick={() => someFunction()}
someFunction is called immediately when the onClick event is triggered.

I didn't got it to be honest.