lazy promises

I would love promises to be lazy and not get executed on creation - but instead on then. I know this is not part of the spec - but I still find it very useful.

Seems like there are others interested in this, too. There is a work-around on SO and someone else created a lib for it.

Would love to hear some thoughts on this.

[tjconcept]

I don’t think this would belong anywhere in Bluebird - it’s not really a promise-thing anymore than a general async challenge.

I wrote a package for lazy-execution of an async job (and reuse of it’s promise so that it will never run twice): https://www.npmjs.com/package/smoop#using-promises

Internally it is using Bluebird, but it works with callbacks as well. The async job can be either callback taking or promise returning as well.

Example:

var smoop = require('smoop');

var myQuery = smoop(function(){
    return queryTheDatabase();
});

// return now and queryTheDatabase is never called

myQuery().then(function( result ){
    console.log(result);
});

// queryTheDatabase is called

It is pretty simple (63 SLOC) and has options for expiring the result or continuously refreshing it.

[tcurdt]

@petkaantonov

Instead of passing a lazy promise, you can try passing a function that returns an eager promise which has the same effect.

Not sure I can follow. What’s an “eager promise”? And won’t that also be yet another fresh promise instance on calling it twice?