this context

[A Former User]

Why this context wrong in onreadystatechange?

import QtQuick 2.0

Item {
  id: root

  Component.onCompleted: {
    console.log("Component completed")

    console.log(this.__proto__.constructor.name); // Object

    String.prototype.repeat = function(times) {
      return Array(++times).join(this);
    };

    // works
    console.log("*".repeat(10));

    function Foo(x) {
      this._x = x;
    }

    Foo.prototype.getX = function() {
      return this._x;
    }

    // works
    var foo = new Foo(42);
    new Foo("bar");
    console.log(foo.getX()); // 42

    console.log(this.root); // undefined

    var xhr = new XMLHttpRequest;
    xhr.open("GET", "http://httpbin.org/get");
    xhr.onreadystatechange = function() {
      // wrong this context
      if (this.readyState == this.DONE && this.status == 200) {
        console.log(this === xhr); // false
        console.log(this.response); // undefined
      }
      // it's works
      if (xhr.readyState == xhr.DONE && xhr.status == 200) {
        console.log(xhr.response);
      }
    };
    xhr.send();
  }
}

[A Former User]

Because you are using the this keyword inside the Item element, therefore in your example code, this refers to the Item. That's why console.log(this === xhr); is false. Therefore your last conditional should be like this:

if (xhr.readyState == xhr.DONE && xhr.status == 200) {
            console.log(xhr.response);
}

[A Former User]

Ok.

import QtQuick 2.0
import "assets/scripts/test.js" as Q

Item {
  id: root
  Text {
    text: "Test!"
  }
}

assets/scripts/test.js

(function() {
  var xhr = new XMLHttpRequest;
  xhr.open("GET", "http://httpbin.org/get?foo=bar");
  xhr.onreadystatechange = function() {
    console.log(this == xhr);
  };
  xhr.send();
})();

Console output:

Запускается C:\Qt\5.5\mingw492_32\bin\qmlscene.exe...
qml: false
qml: false
qml: false
C:\Qt\5.5\mingw492_32\bin\qmlscene.exe завершился с кодом 0