In UI5, you can implement a promise using the jQuery Promises library. Here's an example of how to create and use a promise in UI5:

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var oDeferred = jQuery.Deferred();

// Perform some async operation (e.g. making an HTTP request)// and resolve or reject the promise based on the result
$.ajax({
  url: "/path/to/api",
  success: function(data) {
    oDeferred.resolve(data); // resolve the promise with the data
  },
  error: function() {
    oDeferred.reject(); // reject the promise
  }
});

// Use the promise
oDeferred.promise().done(function(data) {
  // Handle the resolved promise with the data
}).fail(function() {
  // Handle the rejected promise
});

In this example, we create a new Deferred object and perform an async operation (an HTTP request) using jQuery's $.ajax() method. We resolve or reject the promise based on the result of the async operation. Finally, we use the promise() method to get a reference to the promise object and attach callbacks using the done() and fail() methods.

Note that jQuery Promises follow the Promises/A+ specification, so you can use other Promise implementations in UI5 as well.