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?
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.