React.js: loading JSON data with Fetch API and props from object array

Question

Totally new to react.js and after going through the tutorial and reading the docs, I\'m still struggling a bit with using js fetch to load my data from a JSON file as well as se

Answer 1

Ok... Here's the working project. Got some help from @gumingfeng and @hkal.

Lessons learned:

1. The React docs are ridiculously outdated.
2. Straight JS vs. JSX is really no worse imo.
3. Corrected some syntax errors in Fetch.
4. Object array needs to be instantiated after data is loaded in order to pass DOM references in the constructors.
5. However, calling setState() inside componentDidUpdate() triggers an infinite loop so had to set the object array directly and independently from state.
6. When creating DOM elements from an array, React does not automatically assign the event handler to the particular element. In other words, it must be passed the array index so that can be used to access its values in the arrays.

Whew, I think that's it. Hope this helps others.

And I would just conclude by saying, give React a try without JSX. It's really not that bad :)

const { Component } = React;
const { render } = ReactDOM;

class CanvasAnimation extends Component {
    
    constructor(){
        super();
        this.state = {
            data: []
        };
    };
    
    componentDidMount() {
        fetch("data.json")
            .then( (response) => {
                return response.json() })   
                    .then( (json) => {
                        this.setState({data: json});
                    });
    };
    
    componentDidUpdate() {
        function animation(json) {
            return json.map( (data) => {
                return(
                    new CanvasSprite(
                        document.getElementById(data.id),
                        data.width,
                        data.height,
                        data.spriteSheetURL,
                        data.rows,
                        data.columns,
                        data.totalFrames)
                );
            });
        };
        //this.setState({animation: animation(this.state.data)}); //causes infinite loop
        this.animation = animation(this.state.data);
    };
    
    handleInteraction(event, index) {
        var offsetY = event.clientY -  document.getElementById(this.state.data[index].id).getBoundingClientRect().top;
        var relY = offsetY/this.state.data[index].height;
        this.animation[index].setFrame(relY);
    };
    
    render() {
        var canvases = this.state.data.map( (data, index) => {
            return (
                React.createElement('canvas', 
                                    {id : data.id,
                                    width : data.width,
                                    height : data.height,
                                    //style : {border: '5px solid white'},
                                    onMouseMove : (event) => this.handleInteraction(event, index)}
                                    )
            );
        });
        return(
            React.createElement('div', null, ...canvases)
        );
    };
    
};
  
    
render(
    React.createElement(CanvasAnimation, null),
    document.getElementById('content')
);

Answer 2

You have tons of syntax errors in your code, I have fixed them for you.

const { Component } = React;
const { render } = ReactDOM;

class CanvasAnimation extends Component {
  state = {
    data: []
  };

  loadData() {
    function animation(json) {
      return json.map(function(data) {
        return (
          new CanvasSprite(
            document.getElementById(data.id),
            data.width,
            data.height,
            data.spriteSheetURL,
            data.rows,
            data.columns,
            data.totalFrames
          )
        );
      });
    }
    fetch("data.json")
      .then(response => response.json())
      .then(json => {
        console.log(json);
        this.setState({
          data: json,
          animation: animation(json)
        });
      });
  }

  componentDidMount() {
    this.loadData();
  }

  handleInteraction(e) {
    var offsetY = e.clientY - e.node.getBoundingClientRect().top;
    var relY = offsetY/this.state.data.height;
    this.props.animation.setFrame(relY);
  }

  render() {
    var canvases = this.state.data.map(function(data) {
      return (
        <canvas
          id={data.id} 
          width={data.width} 
          height={data.height}
          style={{border: '5px white'}}
          onMouseOver={this.handleInteraction}
        />
      );
    });

    return (
      <div>{canvases}</div>
    );
  }
}

render(
  <CanvasAnimation />,
  content
);

I don't know the response of your API so I'm not sure if there's other to fix.

Some of the problems I have noticed:

• Probably your indentation is wrong, because you had functions with double return statements. I suggest you to enable ESLint in your IDE to catch those errors.

• You have not understood how setState works, you can't just do:

  this.setState({
    foo: 'bar',
    baa: myFn(this.state.foo)
  });
  

  Otherwise, this.state.foo inside myFn will refer to the old value of it, and not to the new one that you are setting right now.
  You'd have to do this.setState({foo: 'bar'}, () => this.setState({baa: myFn(this.state.foo)}), but then, it's better to do as I did in the code I have fixed above.