What is sync in Backbone.js?

Sync is a function that is called every time. It attempts to read or save a model to the server. It persists the state of the model to the server.

In Backbone.js, sync is a method that handles the communication between your application and a server. It is responsible for the CRUD (Create, Read, Update, Delete) operations on Backbone models and collections. The sync method abstracts the underlying AJAX (Asynchronous JavaScript and XML) requests and provides a consistent interface for interacting with your server-side data.

When you define a Backbone model or collection, you can specify a url property. Backbone will use this URL to perform AJAX requests whenever you create, read, update, or delete instances of that model or collection.

The default implementation of sync in Backbone.js uses jQuery.ajax to perform these HTTP requests, but you can override sync with your own implementation if you need to customize the behavior.

Here’s a brief overview of the CRUD operations and how they map to sync:

1. Create: When you create a new model instance and call save() on it, Backbone will send a POST request to the server with the model data.
2. Read: When you fetch a model or collection using fetch(), Backbone will send a GET request to the server to retrieve the data.
3. Update: When you call save() on an existing model instance that has already been fetched from the server, Backbone will send a PUT request to update the data on the server.
4. Delete: When you call destroy() on a model instance, Backbone will send a DELETE request to remove the data from the server.

By default, Backbone.js’s sync method is asynchronous, meaning it doesn’t block the execution of other code while waiting for the AJAX request to complete. However, you can pass { async: false } as an option to make the request synchronous if needed, though this is generally discouraged due to potential performance and usability issues.