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Home Interviews Error handling and retries with AJAX requests in Knockout – MindStick
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ICSM Computer
ICSM Computer 23 Apr 2025
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Handling errors and retries in AJAX requests is crucial for making your Knockout.js apps robust, especially when dealing with flaky connections or server issues. Let’s go over a clean and reusable way to do this using observables, retry logic, and error UI feedback. 

Basic Setup with Observables

function DataViewModel() {
  const self = this;

  self.data = ko.observable(null);
  self.error = ko.observable('');
  self.loading = ko.observable(false);
  self.retryCount = ko.observable(0);
  const MAX_RETRIES = 3;

  self.loadData = function () {
    self.loading(true);
    self.error('');

    $.ajax({
      url: '/api/data',
      method: 'GET',
      success: function (response) {
        self.data(response);
        self.retryCount(0); // reset retry count on success
      },
      error: function (xhr, status, err) {
        self.retryCount(self.retryCount() + 1);

        if (self.retryCount() < MAX_RETRIES) {
          console.warn(`Retry ${self.retryCount()}...`);
          setTimeout(self.loadData, 1000); // retry after 1s
        } else {
          self.error("Failed to load data after several attempts.");
        }
      },
      complete: function () {
        self.loading(false);
      }
    });
  };

  self.retryNow = function () {
    self.retryCount(0);
    self.loadData();
  };

  self.loadData(); // Initial load
}

ko.applyBindings(new DataViewModel());

HTML Template Example

<div data-bind="if: loading">
  <p>🔄 Loading data...</p>
</div>

<div data-bind="if: error">
  <p style="color: red;" data-bind="text: error"></p>
  <button data-bind="click: retryNow">Retry</button>
</div>

<div data-bind="if: data">
  <pre data-bind="text: ko.toJSON(data, null, 2)"></pre>
</div>

Best Practices

Tip Why
Use .retryCount() To control retry attempts
setTimeout() Avoid hammering the server immediately
self.error() Show user-friendly messages
Wrap $.ajax() Use a custom utility function for reuse

Optional: Create a Reusable ajaxWithRetry Utility

function ajaxWithRetry(options, maxRetries = 3, delay = 1000) {
  let attempts = 0;

  function attemptAjax(resolve, reject) {
    $.ajax({
      ...options,
      success: resolve,
      error: function (xhr, status, err) {
        if (++attempts < maxRetries) {
          setTimeout(() => attemptAjax(resolve, reject), delay);
        } else {
          reject(err);
        }
      }
    });
  }

  return new Promise((resolve, reject) => {
    attemptAjax(resolve, reject);
  });
}

Now use it like this in your ViewModel:

self.loadData = function () {
  self.loading(true);
  self.error('');

  ajaxWithRetry({ url: '/api/data', method: 'GET' })
    .then(data => self.data(data))
    .catch(() => self.error("Failed after multiple retries"))
    .finally(() => self.loading(false));
};
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