Reference: jQuery Deferred Object

A deferred object registers callback functions that can be called when an asynchronous process is completed.

Parameter

1. beforeStart - A function that is called just before the constructor returns.

Syntax

$.Deferred(beforeStart);

Example

Get a list of employees whose given name is "John"

function getEmployees(givenName) {
  return $.Deferred(function (deferred) {
    // Get the list of employees from the server.
    $.getJSON("/employee/list", function (list) {
      var employees = [];
 
      // Loop over the list of employees. If the employee givenName matches the givenName passed in, then
      // add the employee to the array of employees to be returned.
      for (var idx = 0; idx < list.length; idx++) {
        if (list[idx].givenName === givenName) {
          employees.push(list[idx]);
        }
      }
 
      // Resolve with the list of employees with the given name that was passed in.
      deferred.resolve(employees);
    });
  });
}
 
getEmployees("John").then(function (employees) {
  for (var idx = 0; idx < employees.length; idx++) {
    console.log(JSON.stringify(employees[idx]));
  }
});