sam-w · August 5, 2019 03:16 · Aug 5, 2019 · Aug 5, 2019
diff --git a/Promise.firstSuccess → Promise.firstSuccess.ts b/Promise.firstSuccess → Promise.firstSuccess.ts
diff --git a/Promise.firstSuccess b/Promise.firstSuccess
@@ -0,0 +1,32 @@
+/**
+ * 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),
+    );
+}
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