Handle backpressure

tests/demux.spec.ts

@@ -107,3 +107,58 @@ test.cb("should spread per key using keyBy", t => {
     input.forEach(event => demuxed.write(event));
     demuxed.end();
 });
+
+test.cb("should emit errors", t => {
+    t.plan(2);
+    const input = [
+        { key: "a", val: 1 },
+        { key: "b", val: 2 },
+        { key: "a", val: 3 },
+        { key: "a", val: 4 },
+    ];
+    const results = [
+        { key: "a", val: 2 },
+        { key: "b", val: 3 },
+        { key: "a", val: 4 },
+        { key: "a", val: 5 },
+    ];
+    const destinationStreamKeys = [];
+    const sink = new Writable({
+        objectMode: true,
+        write(chunk, enc, cb) {
+            expect(results).to.deep.include(chunk);
+            expect(input).to.not.deep.include(chunk);
+            t.pass();
+            cb();
+        },
+    }).on("unpipe", e => console.log("sink err"));
+
+    const construct = (destKey: string) => {
+        destinationStreamKeys.push(destKey);
+        const dest = map((chunk: Test) => {
+            if (chunk.key === "b") {
+                throw new Error("Caught object with key 'b'");
+            }
+            return {
+                ...chunk,
+                val: chunk.val + 1,
+            };
+        }).on("error", e => console.log("got err"));
+
+        dest.pipe(sink);
+        return dest;
+    };
+
+    const demuxed = demux(
+        construct,
+        { keyBy: (chunk: any) => chunk.key },
+        { objectMode: true },
+    );
+    demuxed.on("error", e => {
+        expect(e.message).to.equal("Caught object with key 'b'");
+        t.pass();
+        t.end();
+    });
+    input.forEach(event => demuxed.write(event));
+    demuxed.end();
+});