React passing parameter with arrow function in child component

Question

I have these parent and child component, I want to pass click function to select an item in child component. Yet it seems the function in child component become automatically called instead of waiting until the user click the element. To make it clearer here is my parent and child components

export class ParentView extends Component {
  state = {
    selectedItem: {}
  }

  handleClick = (item) => {
    alert('you click me');
    this.setState({selectedItem: item});
  } 

  render() {
    let item = { name: 'Item-1' };
    return (
      <div>
        <ChildItem item={item} handleClick={this.handleClick} />
      </div>
    );
  }
}

export class ChildItem extends Component {
  render() {
    const {item, handleClick} = this.props;
    return (
      <div>
        <a  onClick={handleClick(item)} />
      </div>
    );
  }
}

Those are my components using arrow function to pass handleClick to child component, yet alert always being called at first render without being triggered by user. Any suggestion?

Answer 1

You should pass a function itself to onClick, not a result of the passed function invocation.

If you would like to invoke it with param, you have options:

• bind it with item with handleClick.bind(this, item). bind creates a new function will have a predefined first parameter - item
• pass new arrow function like () => handleClick(item)

An example below:

export class ChildItem extends Component {
  render() {
    const { item, handleClick } = this.props;

    return (
      <div>
        <a onClick={() => handleClick(item)} />
      </div>
    )
  }
}

In your code you're invoking a function in onClick declaration, so the result of handleClick execution will be passed to onClick, what is most likely not something you wanted to achieve.

<a onClick={handleClick(item)} />

Update:

as @dhilt wrote, there is a drawback of such approach. Since the newly created arrow function and .bind also creates new function every time the render method of ChildItem is invoked, react will threat the resulted react element as a different, comparing to the previous "cached" result of render method, that means that likely it might lead to some performance problems in the future, there is even a rule regarding this problem for eslint, but you shouldn't just follow this rule because of two points.

1) performance problems should be measured. we don't forbid using Array.prototype.forEach in favor of a regular for because for is the same or "faster".

2) definition of click handlers as class properties leads to increasing of the initializing step of the component instance. Re-render is fast and efficient in react, so sometimes the initial rendering is more important.

Just use what's better for you and likely read articles like this https://cdb.reacttraining.com/react-inline-functions-and-performance-bdff784f5578