Promise that returns second promise result if first fails

Question

I would like to return the value of a second promise if the first (value in cache) fails.

I have the following code, but resolve is not defined.

exports.getConfig = function (a, r) {
  return new Promise(resolve, reject)    {
    getConfigFromCache(a, r)
        .catch(function(e){
            getRouteConfigFromWeb(a, r)
        }).then(function(result) {
            //return value of the promise that was called
            resolve(result)
        })
  }
};

Assume that both getConfigFromCache and getRouteConfigFromWeb return promises correctly.

Is there a way to accomplish this, or am I thinking through it incorrectly?

thanks for pointing that out. I still have the same issue when calling resolve(result) — Tony Laidig, Apr 27 '16 at 20:23
try caching `resolve`. `var _resolve = resolve;` or something similar before you enter `getConfig` — Ryan, Apr 27 '16 at 21:34
is the `.then()` supposed to be returning the result of either `getConfigFromCache` _or_ ``getRouteConfigFromWeb` regardless of which is used? — Alnitak, Apr 27 '16 at 21:41
Avoid the [`Promise` constructor antipattern](http://stackoverflow.com/q/23803743/1048572)! Just `return` the promises. — Bergi, Apr 27 '16 at 22:25

Alnitak · Accepted Answer · 2016-04-27T21:55:30.917

3

You shouldn't need to create a new Promise at all:

exports.getConfig = function (a, r) {
    var cache = getConfigFromCache(a, r);
    return cache.catch(function(e) {
        return getRouteConfigFromWeb(a, r);  // NB: return *essential*
    });
}

If the getConfigFromCache() call succeeds, the resulting resolved Promise should skip through the .catch and get returned directly.

If the cache call fails, the Promise returned from getRouteConfigFromWeb() is returned instead.

I note also that the very first line of your question actually gives the solution: "I would like to return the value of a second promise if the first (value in cache) fails." - you never actually put a return in the .catch block!

edited Apr 27 '16 at 21:55

answered Apr 27 '16 at 21:43

Alnitak

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That is correct. The promise was passed through after all. – Tony Laidig Apr 27 '16 at 21:46
@user1886721 the key thing you were missing in your original code was the `return` inside the `.catch` block. Without it, the Promise chain is broken. – Alnitak Apr 27 '16 at 21:47

Promise that returns second promise result if first fails

1 Answers1