Correct Use of Arrow Functions in React

Correct use of arrow functions in React

I understand that arrow functions make things more efficient by not
recreating the functions each render similar to how binding in the
constructor works.

This is not true. It depends on where exactly are you using the Arrow function. If Arrow function are used in render method, then they create a new instance everytime render is called just like how bind would work. Consider this example

<div onClick={()=>{this.onClick()}}>Previous</div>

Here each time render is called an anonymous function is created and that function when called, calls this.onClick.

However consider the case below

onClick = () => {
    console.log("Div is clicked")
}

In above case, the arrow function does not recreate function everytime, but binds the context to the React component as An arrow function does not have its own this; the this value of the enclosing execution context is used. once when the class is instantiated. This is similar to how binding works is constructor. This is a part of proposed class fields for arrow functions and it isn't a ES6 feature,

To understand what you wish to ask, you must know that a function gets its context from where it is called. Check this question for more understanding.

In your case, you have used Arrow function to define prevItem and hence it gets the context of the enclosing React component.

prevItem = () => {
    console.log("Div is clicked")
}

render(){
    return (
         <SecondClass prevItem={this.prevItem} />
    )
}

Now in its child, even if you call prevItem with any custom context, using bind or arrow function, prevItem when executed in parent i.e Main.js will get the context of its enclosing React component. And since you just wish to execute prevItem function and do not want to pass any data to this from the child, writing

<ThirdClass type="prev" onClick={()=>this.props.prevItem()} />

and

<div onClick={()=>{this.props.onClick()}}>Previous</div>

is simply useless and will only add to performance implication since new functions are created in SecondClass and ThirdClass everytime. You simply don't need to have these functions defined as arrow function and could just write

<ThirdClass type="prev" onClick={this.props.prevItem} />

and

<div onClick={this.props.onClick}>Previous</div>

since its already binded in the parent.

Now even if you have to pass some additional data to these function from ThirdClass and SecondClass, you shouldn't directly use Arrow function or bind in render. Have a look at this answer on How to Avoid binding in Render method

Arrow Functions in React/JavaScript

To make it clear, this is not React-specific behavior; it is a part of how functions work in JavaScript. Generally, if you refer to a method without () after it, such as onClick={this.handleClick}​, you should bind that method.

If calling bind annoys you, there are two ways you can get around this. If you are using the experimental public class fields syntax, you can use class fields to correctly bind callbacks.

If you aren’t using class fields syntax, you can use an arrow function in the callback.

The problem with arrow function syntax is that a different callback is created each time the button renders. In most cases, this is fine. However, if this callback is passed as a prop to lower components, those components might do an extra re-rendering. We generally recommend binding in the constructor or using the class fields syntax, to avoid this sort of performance problem.

You can read more about this on their documentation here:

https://reactjs.org/docs/handling-events.html

Bind vs Arrow Function when passing onChange props (React)

Since it is only using a reference to the function, we are not causing a re-render each time

You're wrong.

The bind method returns a new function (so will trigger a re-render).

---

To avoid re-renders, make use of the useCallback hook.

Or, since you don't appear to be making use of this inside myFunction (otherwise () => myFunction() would break it), just pass myFunction itself without creating a new function.

onClick={myFunction}

React: Which is recommended arrow or normal function?

There are so many answers around there but people always get confused. I know this because I got confused once a while ago. After some time, I grasped the concepts.

1. Bind object/function manually in order to play with state or props
   inside the function and to avoid scope-related issues

Not exactly true. You don't need to bind the functions to play with state or props. You bind the function to this when you lose this context in the scope. For example in a callback function.

class App extends React.Component {
  state = {
    name: "foo",
  }
  aFunction() {
    console.log( this.state.name );
  }
  render() {
    return <div>{this.aFunction()}</div>;
  }
}

You don't need to bind your function since this points your class and you don't lose its context. But if you use your function in a callback like a button, you have to bind it:

class App extends React.Component {
  state = {
    name: "foo",
  }
  aFunction() {
    console.log( this.state.name );
  }

  render() {
    return (
      <div>
        <button onClick={this.aFunction}>Click</button>
      </div>
    );
  }
}

This does not work since you lose the context. Now, you need to get its context back somehow right? Ok, let's see how we can do this. First, I want to bind it in the button callback.

<button onClick={this.aFunction.bind(this)}>Click</button>

Yeah, this works. But, it will be recreated in every render. So:

2. Bind object/function always in constructor but not directly in render

Yes. Do not bind it like I did above, do it in your constructor.

3. If you do it in constructor then Webpack creates new object/function in bundle.js file only once when your component
   renders for the first time

4. If you do it directly in render then Webpack will create a new object/function in bundle.js file every time your component renders
   and re-render

You are summarizing here what I've tried to explain up to now. But, I suppose Webpack is not the one doing this, your App is.

5. If you don’t bind then you can’t access state or props. You have to assign current object to a local variable otherwise this.state or
   this.props is undefined

Again, if you use your function inside your class scope, you don't have to bind it. If you use this function outside of your class, like a button callback, you have to bind it. This is not related to state or props. This is related to using this.

Your second option for binding is doing the binding in the constructor by using a regular function and the third one is using an arrow function without binding.

Now, arrow functions.

1.No need to bind an object/function in constructor nor render

Yes.

2. You no need to depend on local variable interms of current object i.e., let that = this;

Yes.

3. You will not have scope issues and object/function binding takes automatically

Yes.

But my query is that I heard that it’s recommended to use normal
function and bind it in constructor rather than using arrow function
because arrow functions create new object/function in Webpack
bundle.js every time your component renders & re-renders.

Like everybody said, that depends on where you use them.

render() {
    return (
        <div>
            <button onClick={() => this.aFunction()}>Click</button>
        </div>
    );
}

Here, it will be recreated in every render. But if you don't need to pass any argument to it, you can use it by reference.

render() {
    return (
        <div>
            <button onClick={this.aFunction}>Click</button>
        </div>
    );
}

This works as the previous one. So, if you see a () in your render method, this function recreated in every render. Regular or an arrow one, doesn't matter. If you are invoking it somehow, then you are recreating it. This applies to bind in the render like aFunction.bind(this). I see () there.

So, use functions by their references to avoid this issue. Now, the big question is what happens when we need some arguments? If you use an arrow function to pass an argument then try to change your logic.

But is it really important as much? Like @Eric Kim said, optimizing is an issue if you really need it. This is a general suggestion since I've heard this from lots of people. But personally, I am trying to avoid using functions if they will be recreated in every render. But again, this is totally personal.

How can you change your logic? You are mapping over an array with an item and creating
a button. In this button, you are using a function that passes item's name to a function.

{
    items.map( item =>
        <button onClick={() => this.aFunction(item.name)}>Click</button>
    )
}

This function will be recreated in every render for each item! So, change your logic, create a separate Item component and map it. Pass the item, aFunction as props. Then with a handler function in this component use your function.

const Item = ( props ) => {
    const handleClick = () => props.aFunction( props.item.name );
    return (
        <button onClick={handleClick}>Click</button>
    );
}

Here, you are using an onClick handler with its reference and it invokes your real function. No function will be recreated in every render. But, as a downside, you need to write a separate component and a little bit more code.

You can apply this logic most of the time. Maybe there will be some examples you can't, who knows. So the decision is yours.

By the way, the Medium post that @widged gave in the comments is a famous discussion about this issue. Are arrow functions really slower than the regular ones? Yes. But how much? Not so much I guess. Also, this is true for transpiled code. In the future when they become native, then they will be the faster ones.

As a personal side note. I was using arrow functions all the time since I like them. But a while ago in a discussion, someone said

When I see an arrow function in the class I think that: 'This function
is being used/called outside of this class'. If I see a regular one I
understand that this function called inside the class.

I really liked this approach and now if I don't need to call my function outside of my class I am using a regular one.

React.js event with arrow function vs without any () function

component expects onChange as a function, that function will be called when the user changes the value of the input.

NOTE: It's syntactically correct but the handle function will not execute.

When you do

<input onChange={handle}></input>
// it's equivalent to
<input onChange={(e)=>{ setText(e.target.value); }}></input>

But when you do

<input onChange={()=>handle}></input>
// it's equivalent to
<input onChange={(e) => ()=>{ setText(e.target.value);}}></input>

Why does arrow function work but regular function does not React JS

You need to bind this if you are going to use them. Arrow functions do not need binding because arrow function does not have the following in its context:

• this

• arguments

• super

• new.target

  constructor(props) {
    super(props);
    this.state = { content: "test" };
    this.changer = this.changer.bind(this);
  }
  
  changer() {
    this.setState({ content: "changed" });
  }
  render() {
    return (
      <div>
        <h1>{this.state.content}</h1>
        <button onClick={this.changer}>click me to change</button>
      </div>
    );
  }
  

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