First fulfilled promise by array order

Competitive races like when.any and when.race are primarily for determining the first promise to fulfill or reject in time/causal order. If you need to find the first promise to fulfill in array index order, you can use recursion, similar to the following:

// Returns the fulfilled promise with the lowest array index.
// If all promises in the array reject, returns a rejected promise
// whose reason is an Error with an `.errors` Array property containing
// all the rejection reasons.
function findFirst(promises){
	return recurseFindFirst(0, [], promises);
}

function recurseFindFirst(i, errors, promises) {
	if(i === promises.length) {
		var e = new Error('All promises rejected');
		e.errors = errors;
		return when.reject(e);
	}

	return when(promises[i]).catch(function(e) {
		errors.push(e);
		return recurseFindFirst(i+1, errors, promises);
	});
}

Here's an example usage. Note that this logs 'c', since it has a lower index that 'd', even though 'd' fulfills first in time. In contrast, when.any would log 'd', and when.race would reject with Error('a').

var testList = [
	when.reject(new Error('a')),
	when.reject(new Error('b')),
	when.resolve('c').delay(1000),
	when.resolve('d')
];

findFirst(testList).done(console.log.bind(console)); // Logs 'c'