Lazy promises

This library achieves lazy evaluation when using promises. This is necessary when statements with side-effects are used with promises.

I want to achieve something that is equivalent to this p1() && p2() with promises (here is p2 only evaluated if p1 is false).

Here is an example:

> P.deferred(console.log, this, 'hi')
{ then: [Function] }
> P.deferred(console.log, this, 'hi').then()
hi

deferred takes the arguments funcArg, thisArg and the arguments that should be passed to funcArg. deferred.then will simply do funcArg.apply(thisArg, arguments)

On the other hand, these two statements behave exactly the same way:

> Promise.resolve(console.log('hi'));
hi
{}
> Promise.resolve(console.log('hi')).then()
hi
{}

Lazy promises functions depend on the NodeJS promise library but should work with any Promise/A+ compatible framework.

The library also contain a number of functions for conditional evaluation of promises. Here are some examples of how the library is used.

like if(b()) { p() }. deferred ensures that p is evaluated only if b is true:

promiseIf(deferred(false), deferred(true) )
.then(function(x) {
  assert.equal(x, null, 'promiseIf should be null');
}).catch(console.log)

like p1() && p2(). deferred ensures that p2 only is evaluated if p1 is true:

promiseAnd(deferred(true), deferred(true))
.then(function(x) {
  assert.equal(x, true, 'promiseAnd should be true');
}).catch(console.log)

like p1() || p2(). deferred ensures that p2 only is evaluated if p1 is false:

promiseOr(deferred(false), deferred(true))
.then(function(x) {  
  assert.equal(x, true, 'promiseOr should be true');
}).catch(console.log)

like !p():

promiseNot(deferred(null))
.then(function(x) {
  assert.equal(x, true, 'promiseNot should be true');
}).catch(console.log)

Contributions & License

All feedback is greatly appreciated! Just create a github issue or fork request.

Thank's to Forbes Linsey for valuable input.

License: MIT