对象属性始终未定义

Question

This is the first time I've used JS objects and I'm confused as to why this property is always undefined:

function Rotator() {
    this.interval = 300;
    this.image = 0;
    this.images = undefined;
}

Rotator.prototype.Fetch = function(links) {
    console.log("Fetch called");
    this.images = links;
}

Rotator.prototype.Current = function() {
    if (this.images == undefined) {
        console.log("Error, images is undefined");
    }
    return this.images[this.image];
}

r = new Rotator;
$.getJSON("./data.php", function (data) {
    r.Fetch(data.images);
});

console.log(r.Current());

The error I get is:

Uncaught TypeError: Cannot read property '0' of undefined

The JSON returned is working fine, and fetch is marked as called in the console (when logged the data is fine as well). Why is Rotator.images always undefined?

Edit: Some console.log results:

• Logging data.images in $.getJSON results in correct data.
• Logging links in Fetch results in correct data.
• Logging this.images in Fetch results in correct data.
• Logging this.images in Current results in null.

Answer 1

Because getting JSON is asynchronous, that's why the data is only available in the callback function.

$.getJSON("./data.php", function (data) { // callback function
    r.Fetch(data.images); // this will run when the data is available
});

console.log(r.Current()); // this will run immediately -> data.images is null

Everything that depends on the data should be placed in the callback function!

Answer 2

You can't use undefined that way. Use null instead:

this.images = null;

and

if (this.images == null) {

Edit:

You also have to avoid using the images property if it's null:

Rotator.prototype.Current = function() {
  if (this.images == null) {
    console.log("Error, images is undefined");
    return null;
  }
  return this.images[this.image];
}

Answer 3

Will this get me purists on my neck or is it acceptable?

Rotator.prototype.Current = function() {
    if (this.images) return this.images[this.image];
    console.log("Error, images is undefined, null, empty or 0");
}