Error encountered while trying to call callback functions

Question

Error encountered while trying to call callback functions

I encountered an error in my code, but I managed to resolve it independently. Could someone please provide an explanation of why the code wasn't working and delve into the mechanics behind the issue?

Here is the code snippet:

var listTables = function (tables) {
  console.log(tables);
}

var ajaxReq = function (success_cb, url) {
  // success_cb can be invoked here without errors
  var http = new XMLHttpRequest();
  http.onreadystatechange = function (success_cb) {
    if (this.readyState == 4 && this.status == 200) {
      success_cb(this); // Uncaught TypeError: success_cb is not a function
    }
  }
  http.open("GET", url, true);
  http.send();
}
ajaxReq(listTables, "http://www.someurl.com");

Upon removing the success_cb callback from the onreadystatechange function definition, the code ran smoothly. Is this issue related to scope?

javascript ajax xmlhttprequest onreadystatechange

Answer 1

Answer №1

The issue lies in the repetition of success_cb as a parameter within the inner function:

var ajaxReq = function (success_cb, url) { // The first usage of success_cb parameter is correct.
  var http = new XMLHttpRequest();
  http.onreadystatechange = function (success_cb) { // The second usage is incorrect!
    // ...
  }
  // ...
}

This leads to two unintended consequences:

It overshadows the original (first) success_cb, making it inaccessible.
It assigns the event argument of the readystatechange event to success_cb which results in an object.

“Overshadowing” occurs when a variable (or parameter) of the same name is provided at a closer scope than the previous one.

To resolve this issue, simply eliminate the second instance of usage:

var listTables = function (tables) {
  console.log(tables);
}

var ajaxReq = function (success_cb, url) {
  // success_cb can now be invoked here without any errors
  var http = new XMLHttpRequest();
  http.onreadystatechange = function () {
    if (this.readyState == 4 && this.status == 200) {
      success_cb(this);
    }
  }
  http.open("GET", url, true);
  http.send();
}
ajaxReq(listTables, "http://www.someurl.com");

Answer 2

The issue lies in the repetition of success_cb as a parameter within the inner function:

var ajaxReq = function (success_cb, url) { // The first usage of success_cb parameter is correct.
  var http = new XMLHttpRequest();
  http.onreadystatechange = function (success_cb) { // The second usage is incorrect!
    // ...
  }
  // ...
}

This leads to two unintended consequences:

It overshadows the original (first) success_cb, making it inaccessible.
It assigns the event argument of the readystatechange event to success_cb which results in an object.

“Overshadowing” occurs when a variable (or parameter) of the same name is provided at a closer scope than the previous one.

To resolve this issue, simply eliminate the second instance of usage:

var listTables = function (tables) {
  console.log(tables);
}

var ajaxReq = function (success_cb, url) {
  // success_cb can now be invoked here without any errors
  var http = new XMLHttpRequest();
  http.onreadystatechange = function () {
    if (this.readyState == 4 && this.status == 200) {
      success_cb(this);
    }
  }
  http.open("GET", url, true);
  http.send();
}
ajaxReq(listTables, "http://www.someurl.com");

Error encountered while trying to call callback functions

Answer №1

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