Feature Request: Add utility function Promise.first()

I’m frequently in a position where I want to say “Try this. If that doesn’t work, try this. If that doesn’t work, try this”. It’d be nice to be able to write:

Promise.first([
    method1,
    method2,
    method3
], {concurrency:1})

It appears that, currently, you can get bluebird to have this desired behavior, but it’s not very clean. I might just be missing a useful pre-existing pattern, though.

[timhuff]

An issue with your solution, though, is that it completely ignores any errors or exceptions coming out of the functions. Also, if the resolved result is, itself, null, then that would be ignored. I ended up just making a helper function for this, written in coffeescript. In case anyone else finds this useful. Considering how nuanced this issue is and the fact that it is a use case that comes up from time to time, I’m going to go ahead and create that issue after all.

Promise.first = (args..., methods=[], thisArg)->
    if methods.length == 0
        return Promise.resolve()
    errors = new Promise.AggregateError
    firstResolvedMethod = (n=0)->
        methods[n].apply thisArg, args
        .error (error)->
            errors.push error
            if n < methods.length-1
                return firstResolvedMethod n+1
            else
                return Promise.reject errors
    return firstResolvedMethod()

[timhuff]

@petkaantonov Thank you! I agree. Your usage of reduce makes the solution succinct and readable enough. While I feel that it’s a fairly common use-case, I don’t think it’s nearly common enough to necessitate an addition to the API. I’m not going to bother creating a new issue.