使用p5.js在JavaScript中未定义JSON属性

Question

I'm just beginning learning to code and this is my first post here so please forgive me if I violate any rules here.... So here is my question, it seems quite simple but I can just not figure out what is the problem. I have a JSON file like this:

{"imgj":
[
    {
        "id":"1",
        "user_id":"1",
        "tag":"siteplan",
        "imageurl":"images/cb.jpg"          
    },
    {
        "id":"2",
        "user_id":"2",
        "tag":"floorplan",
        "imageurl":"images/cbb.jpg"             
    },
    {
        "id":"3",
        "user_id":"1",
        "tag":"section",
        "imageurl":"images/postit.png"          
    },
    {
        "id":"4",
        "user_id":"2",
        "tag":"siteplan",
        "imageurl":"images/avatar_default.jpg"  
    }
]
}

and in my .js file, I'm trying to get data of img from the JSON file, but always failed.

 p.preload=function(){

                      var url ='imglist.json';
                      imglist = p.loadJSON(url);


                       for(var i=0; i<4; i++) {

                        imgurl = imglist.imgj[i].imageurl;

                        img[i]=p.loadImage("imgurl");
                      }

                    };

The result of the console is shown as below:

                      console.log(typeof imglist == 'object'); //return true
                      console.log(imglist); //return {} imgj:(4)[{...},{...},{...},{...}]
                      console.log(imglist.imgj[1]); //return undefined

it seems that my JSON file is successfully read as an object, but the property of it cannot be read. It's really weird.

Answer 1

This should work however it's only fetch workaround. I don't know how to access this not own property made by loadJSON.

p.preload= function(){

      var url ='imglist.json';
      fetch(url)
        .then(data=>data.json())
        .then(imglist=>{
             //imglist = p.loadJSON(url);

             console.log(imglist.imgj[0]); // Object { id: "1", user_id: "1", tag: "siteplan", imageurl: "images/cb.jpg" }
             for(let i = 0;i< imglist.imgj.length;i++) {

                 imgurl = imglist.imgj[i].imageurl;

                 img[i]=p.loadImage(imgurl);
             }});

};

Answer 2

After a bunch of fiddling I believe loadJSON is actually asynchronous behind the scenes, so your for-loop continues to execute before the file is done loading.

You could store the imglist in a global/classwide variable and do the for-loop inside the setup or draw function instead.

var imglist;
var imgs;

p.preload = function() {
  var url = 'imglist';
  imglist = loadJSON(url);
}

p.setup = function() {
  imgs = imglist.map(function(imglistitem) { 
    return loadImage(imglistitem.imageurl); 
  });
}

I am not quite sure how p5.js is wired behind the scenes, but the example from the reference uses the draw() function to pick apart the json: https://p5js.org/reference/#/p5/loadJSON