Difference in using functions in react-native

Question

Lets say I have some component wo which I pass a function.

export default class SomeComp extends Component {
    constructor(props) {
        super(props);
    }

    _someFunc = () => {
        return ...
    }

    render() {
        return (
            <View
                onLayout={this._someFunc()}
            />

OR

            <View
                onLayout={() => {this._someFunc()}}
            />

        )
    }
}

Where is the difference between onLayout={this._someFunc()} and onLayout={() => {this._someFunc()}}?

Did you mean difference between `onLayout={this._someFunc}` and `onLayout={() => {this._someFunc()}}`? — bennygenel, Sep 13 '17 at 08:11
@bennygenel exactly. As I stated in the last sentence of the Q. — four-eyes, Sep 13 '17 at 08:12
No. You didn't. First of all there is a difference between `onLayout={this._someFunc()}` and `onLayout={this._someFunc}`. — bennygenel, Sep 13 '17 at 08:15

Chris · Accepted Answer · 2017-09-13T08:30:21.067

The difference depends really how and when onLayout is triggered.

onLayout={this._someFunc()}

Each time your component is rendered the onLayout prop will get the return value from the this._someFunc() function. In other words, the function will run on every render.
onLayout={() => {return this._someFunc()}} or onLayout={() => this._someFunc()}

Each time your component is rendered the onLayout prop will bind the reference to an anonymous function which contains the call to the _someFunc() function. The latter function doesn't get executed unless onLayout is somehow triggered.

For example, assume the following component:

class MyApp extends React.Component {

  _someFunc = () => {
    return "bar";
  }

  render() {
    return(
      <div>
        <A foo={this._someFunc()} />
        <B foo={() => {return this._someFunc()}} />
      </div>
    );
  }
}

const A = ({foo}) => {
  console.log(foo);  //prints "bar"
  return <div>A</div>;
}

const B = ({foo}) => {
  console.log(foo);  //prints the reference to _someFunc()
  console.log(foo());  //prints "bar"
  return <div>B</div>;
}

ReactDOM.render(<MyApp />, document.getElementById("app"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="app"></div>

In component A the foo prop will get the value of "bar".

In component B the foo prop will be a reference to a function that you can later call. That function will in turn call _someFunc which will return "bar". So if in component B you want to get the value, you need to call it with this.props.foo().

Difference in using functions in react-native

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