The absence of onreadystatechange() on XMLHttpRequest objects is proving to be problematic

Question

The absence of onreadystatechange() on XMLHttpRequest objects is proving to be problematic

(function (updateSend) {

     XMLHttpRequest.prototype.send = function () {

         console.log(this.onreadystatechange); //null
         updateSend.apply(this, arguments);
     };

})(XMLHttpRequest.prototype.send);

Why does this.onreadystatechange evaluate as null? Despite documentation saying it should be of type function.

javascript ajax xmlhttprequest

Answer 1

Answer №1

` element, it is explained that the `onreadystatechange` property exists and that's why it is set to `null`. If it did not exist, it would be `undefined`. The setting of this property depends on whether it has already been defined as a property or not because in this case, it is clearly set to `null`. An example is given where by creating a new `XMLHttpRequest`, setting `onreadystatechange` to an empty function, and then calling the `send()` method will result in the console logging a `function`. On the other hand, if you were to create a new `XMLHttpRequest` without setting `onreadystatechange` and directly call the `send()` method, the console would log `null`. A demonstration of this process can be found at the provided URL. It is clarified that there is no direct correlation between `null` and `undefined`. It is mentioned that when a new `XMLHttpRequest` is created, the `onreadystatechange` property is automatically set to `null`. Users have the freedom to either leave it as such or define it as a function. An update is shared regarding the correct order for creating and sending an XHR request, emphasizing that `onreadystatechange` should be set before calling the `open` method, followed by the `send` method. This ensures that if `onreadystatechange` is actually specified, it will not be `null` and will indeed be a `function`. The discussion further delves into the behavior of jQuery when making AJAX requests. By analyzing the source code, it is revealed that jQuery sets the `onreadystatechange` property to `null` after calling `send()`. This design choice, although unorthodox, may serve a purpose in handling certain scenarios. In conclusion, the reason for getting `null` as the value of `onreadystatechange` when using jQuery is attributed to its timing of setting the property post send operation. This delays the initialization of `onreadystatechange`, causing it to appear as `null` when trying to access it after overriding the `send` method. Additional examples are provided for clearer understanding at the referenced URLs.

Answer 2

` element, it is explained that the `onreadystatechange` property exists and that's why it is set to `null`. If it did not exist, it would be `undefined`. The setting of this property depends on whether it has already been defined as a property or not because in this case, it is clearly set to `null`. An example is given where by creating a new `XMLHttpRequest`, setting `onreadystatechange` to an empty function, and then calling the `send()` method will result in the console logging a `function`. On the other hand, if you were to create a new `XMLHttpRequest` without setting `onreadystatechange` and directly call the `send()` method, the console would log `null`. A demonstration of this process can be found at the provided URL. It is clarified that there is no direct correlation between `null` and `undefined`. It is mentioned that when a new `XMLHttpRequest` is created, the `onreadystatechange` property is automatically set to `null`. Users have the freedom to either leave it as such or define it as a function. An update is shared regarding the correct order for creating and sending an XHR request, emphasizing that `onreadystatechange` should be set before calling the `open` method, followed by the `send` method. This ensures that if `onreadystatechange` is actually specified, it will not be `null` and will indeed be a `function`. The discussion further delves into the behavior of jQuery when making AJAX requests. By analyzing the source code, it is revealed that jQuery sets the `onreadystatechange` property to `null` after calling `send()`. This design choice, although unorthodox, may serve a purpose in handling certain scenarios. In conclusion, the reason for getting `null` as the value of `onreadystatechange` when using jQuery is attributed to its timing of setting the property post send operation. This delays the initialization of `onreadystatechange`, causing it to appear as `null` when trying to access it after overriding the `send` method. Additional examples are provided for clearer understanding at the referenced URLs.

Answer 3

Answer №2

It is your responsibility to define it yourself, this function will execute when the state changes. It operates similarly to other event handlers such as window.onload, element.onclick, and so on.

The conventional approach involves:

    var request = new XMLHttpRequest();
    request.onreadystatechange = function() {
        if (request.readyState === 4) {
              if (request.status === 200) {
                  // process request.responseText
              }
        }
    };
    request.open('GET', url);
    request.send();

Answer 4

It is your responsibility to define it yourself, this function will execute when the state changes. It operates similarly to other event handlers such as window.onload, element.onclick, and so on.

The conventional approach involves:

    var request = new XMLHttpRequest();
    request.onreadystatechange = function() {
        if (request.readyState === 4) {
              if (request.status === 200) {
                  // process request.responseText
              }
        }
    };
    request.open('GET', url);
    request.send();

The absence of onreadystatechange() on XMLHttpRequest objects is proving to be problematic

Answer №1

Answer №2

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