Can I do a "lazy" promise with Bluebird.js? [closed]

Question

I want a construct similar to promise that waits until then is called before it runs. That is, if I never actually call then, the promise will never run.

Is this possible?

Answer 1

Make a function which creates and returns a promise on first invocation, but returns the same promise on each subsequent invocation:

function getResults() {
  if (getResults.results) return getResults.results;

  getResults.results = $.ajax(...); # or where ever your promise is being built

  return getResults.results;
}

Promises don't work in such a way that they could support lazy loading. Promises are created by asynchronous code in order to communicate a result. Until the async code has been invoked, there simply is no promise.

You could certainly write a promise-like object which did lazy invocation, but code that generated these promises would be very different:

// Accepts the promise-returning function as an argument
LazyPromise = function (fn) {
  this.promise = null;
  this.fn = fn
}

LazyPromise.prototype.then = function () {
  this.promise = this.promise || fn();
  this.promise.then.apply(this.promise, arguments)
}

// Instead of this...
var promise = fn();

// You'd use this:
var promise = new LazyPromise(fn);

It's better in this uncommon use to make the actual creation of the promise lazy (as in either above example), rather than trying to make promises themselves responsible for lazy-evaluation.