Comparing PrevProps in componentDidUpdate

Question

I am trying to detect when a prop has changed inside componentDidUpdate of a mounted component. I have a test (refreshData in the code below) that is working fine. Is it possible to SOMEHOW pass props in a way that aren't detected by componentDidUpdate(prevProps)?

In component.js:

componentDidUpdate(prevProps){

    //works fine
    if ( this.props.refreshData !== prevProps.refreshData ) {
        if ( this.props.refreshData )
            this.refreshData();
    }

    //these two arent calling
    if ( this.props.selectedCountries !== prevProps.selectedCountries ) {
        if ( this.props.selectedCountries )
            console.log('updated selected countries');
    }

    if ( this.props.selectedLocations !== prevProps.selectedLocations ) {
        console.log('updated selected locations');
    }

}

and in App.js passing the props like:

selectLocation = (id, type, lng, lat, polydata, name, clear = false) => {

  //console.log(type);
  //console.log(lng);
  //console.log(lat);
  //console.log(polydata);

  let selectedType = 'selected' + type;
  let previousState = [];

  if (clear) {
    this.setState({
      selectedCountries: [],
      selectedLocations: [],
      selectedServices: [],
      selectedPoints: [],
      mapCenter: [lng, lat],
      locationGeoCoords: [polydata]
    })
    previousState.push(id);

  } else {

    previousState = this.state[selectedType];
    if (previousState.indexOf(id) === -1) {
      //push id
      previousState.push(id);

    } else {
      //remove id
      var index = previousState.indexOf(id)
      previousState.splice(index, 1);
    }
  }

  if (type === "Countries") {

    this.setState({
      selectedCountries: previousState,
      refreshData: true,
    })
  } else if (type === "Locations") {
    this.setState({
      selectedLocations: previousState,
      refreshData: true
    })
  } else if (type === "Points") {
    this.setState({
      selectedPoints: previousState,
      refreshData: true
    })
  }


}


render() {
  return (

    <component
      selectedCountries={this.state.selectedCountries}
      selectedLocations={this.state.selectedLocations}
      refreshData={this.state.refreshData} />
  }
}

Answer 1

Hi :) as noted in my comment, the issue is in your App.js file - you are mutating an array. In other words, when you THINK you are creating a new array of selected countries to pass down, you are actually updating the original array, and so when you go to do a comparison you are comparing the two exact same arrays ALWAYS.

Try updating your App.js like so -

selectLocation = (id, type, lng, lat, polydata, name, clear = false) => {

  //console.log(type);
  //console.log(lng);
  //console.log(lat);
  //console.log(polydata);

  let selectedType = 'selected' + type;
  let previousState = [];

  if (clear) {
    this.setState({
      selectedCountries: [],
      selectedLocations: [],
      selectedServices: [],
      selectedPoints: [],
      mapCenter: [lng, lat],
      locationGeoCoords: [polydata]
    })
    previousState.push(id);

  } else {

    previousState = [].concat(this.state[selectedType]);
    if (previousState.indexOf(id) === -1) {
      //push id
      previousState.push(id);

    } else {
      //remove id
      var index = previousState.indexOf(id)
      previousState.splice(index, 1);
    }
  }

  if (type === "Countries") {

    this.setState({
      selectedCountries: previousState,
      refreshData: true,
    })
  } else if (type === "Locations") {
    this.setState({
      selectedLocations: previousState,
      refreshData: true
    })
  } else if (type === "Points") {
    this.setState({
      selectedPoints: previousState,
      refreshData: true
    })
  }


}


render() {
  return (

    <component
      selectedCountries={this.state.selectedCountries}
      selectedLocations={this.state.selectedLocations}
      refreshData={this.state.refreshData} />
  }
}

The only difference is the line where you set previousState - I updated it to be

previousState = [].concat(this.state[selectedType]);

By adding the [].concat I am effectively creating a NEW array each time and so then when you apply your changes to the array via push/splice you will be only modifying the NEW array. Then the comparison will work properly once you pass it down as props :)

For your reading interest, I found a post that talks about this a bit: https://medium.com/pro-react/a-brief-talk-about-immutability-and-react-s-helpers-70919ab8ae7c