Promise.firstSuccess.ts

/**
 * Executes a list of Promises in parallel and resolves with the first successful resolution.
 *
 * Like `Promise.race` in that it takes a list of Promises and only waits for the first completion, but
 * differs in that it requires a _successful_ resolution before it completes. Only rejects if _all_ children reject.
 *
 * Different
 * @param promises a list of promises to resolve.
 * @param where an optional predicate. Any Promise `p` which resolves but for which the predicate
 * produces an error will behave as `p` instead rejected.
 */
// Do not let tslint convert this to async. It will get it wrong and change the behaviour.
// tslint:disable-next-line: array-type
function firstSuccess<T>(promises: Promise<T>[], predicate?: (value: T) => Error | null): Promise<T> {
    return Promise.all(promises.map((p) => {
        // If a request fails, count that as a resolution so it will keep
        // waiting for other possible successes. If a request succeeds,
        // treat it as a rejection so Promise.all immediately bails out.
        return p.then(
            (val) => {
                const e = predicate && predicate(val);
                return e ? Promise.resolve(e) : Promise.reject(val);
            },
            (err) => Promise.resolve(err),
        );
    })).then(
        // If '.all' resolved, we've just got an array of errors.
        (errors) => Promise.reject(errors),
        // If '.all' rejected, we've got the result we wanted.
        (val) => Promise.resolve(val),
    );
}