ES6 Promise support

[kornelski]

Fixes #722

[kahnvex]

This addresses my compatibility concerns by adding a thenable fallback, but is weird/unexpected from a behavioral standpoint.

Also, I'd suggest returning a promise from the .end() method if there is community support for it. Having the then method fire the request is arbitrary and weird. If the goal is to use real promises, we should use promises as they were intended - then methods should not perform actions other than to return a new promise. You could also add a new function .endPromise(), or something like that to address backward compatibility concerns.

[kornelski]

I agree returning promise from end() would be better.

I also think it should cache the promise, so that var tmp = superagent.get(); tmp.then(); tmp.then() doesn't try to launch the request again.

[kornelski]

BTW: the code for both node and browser happens to be identical. Do you have a preferred way for sharing/reusing it?

[euwest]

I also think it should cache the promise, so that var tmp = superagent.get(); tmp.then(); tmp.then() doesn't try to launch the request again.

Yeah, which is further reason for end returning the promise and it(i.e. end) not being a part of then(maybe that's what you were saying?). That way you'd have:

var tmp = superagent.get().end();

tmp.then(); 
tmp.then();

[defunctzombie]

I am gonna say it again. I do not care about supporting promises in this library. The .then hack was to make it a simple thenable and I was fine with that. If you want a promise then use a wrapper.

Yes, we need to document the .then call better.

[vire]

What's the status please, there is an closed PR, but in the latest version of superagent (v1.4.0 and node v0.12.7) is not working as expected (see #722).

[FoxxMD]

@vire It was closed but not merged. @defunctzombie said above that he will not support A+ promises

[vire]

@FoxxMD thanks for clarification. It's a pity indeed to not be complaint with some standard approach.

[euwest]

It's a pity indeed to not be comlpaint with some standard approach.

Superagent is not a promises implementation, though. Request objects just have a then method to make it a little easier to coerce into a promise, given an actual implementation.

[niftylettuce]

Here's a promise implementation (uses ES6/ES7 async/await too, with ES6's new fetch() method) https://github.com/glazedio/frisbee.