diff --git a/package.json b/package.json
index d5a2ee0..58a655f 100644
--- a/package.json
+++ b/package.json
@@ -23,7 +23,7 @@
     },
     "scripts": {
     "test": "NODE_PATH=src node node_modules/.bin/ava 'tests/*.spec.ts' -e",
-    "test:debug": "NODE_PATH=src node inspect node_modules/ava/profile.ts",
+    "test:debug": "NODE_PATH=src node inspect node_modules/ava/profile.js",
     "test:all": "NODE_PATH=src node node_modules/.bin/ava",
     "lint": "tslint -p tsconfig.json",
     "validate:tslint": "tslint-config-prettier-check ./tslint.json",
diff --git a/src/functions/baseFunctions.ts b/src/functions/baseFunctions.ts
index 005aa3a..6ff480a 100644
--- a/src/functions/baseFunctions.ts
+++ b/src/functions/baseFunctions.ts
@@ -20,3 +20,4 @@ export { split } from "./split";
 export { stringify } from "./stringify";
 export { unbatch } from "./unbatch";
 export { compose } from "./compose";
+export { demux } from "./demux";
diff --git a/src/functions/compose.ts b/src/functions/compose.ts
index fea69eb..ee17101 100644
--- a/src/functions/compose.ts
+++ b/src/functions/compose.ts
@@ -74,8 +74,10 @@ export class Compose extends Duplex {
         return (this.last as NodeJS.ReadableStream).pipe(dest);
     }
 
-    public write(chunk: any) {
-        return (this.first as NodeJS.WritableStream).write(chunk);
+    public _write(chunk: any, encoding: string, cb: any) {
+        const res = (this.first as NodeJS.WritableStream).write(chunk);
+        cb();
+        return res;
     }
 
     public bubble(...events: string[]) {
diff --git a/src/functions/demux.ts b/src/functions/demux.ts
new file mode 100644
index 0000000..ef7a26a
--- /dev/null
+++ b/src/functions/demux.ts
@@ -0,0 +1,91 @@
+import { WritableOptions, Writable } from "stream";
+
+/**
+ * Return a Duplex stream that is pushed data from multiple sources
+ * @param streams Source streams to multiplex
+ * @param options Duplex stream options
+ */
+export function demux(
+    construct: () => NodeJS.WritableStream | NodeJS.ReadWriteStream,
+    demuxBy: { key?: string; keyBy?: (chunk: any) => string },
+    options?: WritableOptions,
+): Writable {
+    return new Demux(construct, demuxBy, options);
+}
+
+class Demux extends Writable {
+    private streamsByKey: {
+        [key: string]: {
+            stream: NodeJS.WritableStream | NodeJS.ReadWriteStream;
+            writable: boolean;
+        };
+    };
+    private demuxer: (chunk: any) => string;
+    private isWritable: boolean;
+    private nonWritableStreams: Array<string>;
+    private construct: (
+        destKey?: string,
+    ) => NodeJS.WritableStream | NodeJS.ReadWriteStream;
+    constructor(
+        construct: (
+            destKey?: string,
+        ) => NodeJS.WritableStream | NodeJS.ReadWriteStream,
+        demuxBy: { key?: string; keyBy?: (chunk: any) => string },
+        options?: WritableOptions,
+    ) {
+        super(options);
+        if (demuxBy.keyBy === undefined && demuxBy.key === undefined) {
+            throw new Error(
+                "keyBy or key must be provided in second parameter",
+            );
+        }
+        this.demuxer = demuxBy.keyBy || ((chunk: any) => chunk[demuxBy.key!]);
+        this.construct = construct;
+        this.streamsByKey = {};
+        this.isWritable = true;
+    }
+
+    public _write(chunk: any, encoding: string, cb: any) {
+        const destKey = this.demuxer(chunk);
+        if (this.streamsByKey[destKey] === undefined) {
+            this.streamsByKey[destKey] = {
+                stream: this.construct(destKey),
+                writable: true,
+            };
+        }
+        // Throttle when one stream is not writable anymore
+        // Set writable to false
+        // keep state of all the streams, if one is not writable demux shouldnt be writable
+        // Small optimization is to keep writing until you get a following event to the unwritable destination
+
+        let res = false;
+        if (this.isWritable && this.streamsByKey[destKey].writable) {
+            res = this.streamsByKey[destKey].stream.write(chunk, encoding, cb);
+        } else if (this.isWritable) {
+            this.isWritable = false;
+            // Buffer chunk?
+            return this.isWritable;
+        }
+
+        /* If write above returns false and the stream written to was writable previously, we need to make demux
+         * non-writable and update state to know the stream is nonWritable.
+         * If write returns true and the stream was previously not writable, we need to update which streams
+         * are non writable and determine if it is safe for demux to become writable (all streams are writable)
+         */
+        if (!res) {
+            this.streamsByKey[destKey].writable = false;
+            this.nonWritableStreams.push(destKey);
+            this.isWritable = false;
+            this.streamsByKey[destKey].stream.once("drain", () => {
+                this.streamsByKey[destKey].writable = true;
+                this.nonWritableStreams = this.nonWritableStreams.filter(
+                    key => key !== destKey,
+                );
+
+                this.isWritable = this.nonWritableStreams.length === 0;
+            });
+        }
+
+        return this.writable;
+    }
+}
diff --git a/src/functions/index.ts b/src/functions/index.ts
index 2e4b8ad..a5b53db 100644
--- a/src/functions/index.ts
+++ b/src/functions/index.ts
@@ -289,3 +289,13 @@ export function accumulatorBy<T, S extends FlushStrategy>(
 }
 
 export const compose = baseFunctions.compose;
+
+export function demux(
+    construct: (
+        destKey?: string,
+    ) => NodeJS.WritableStream | NodeJS.ReadWriteStream,
+    demuxer: { key?: string; keyBy?: (chunk: any) => string },
+    options?: DuplexOptions,
+) {
+    return baseFunctions.demux(construct, demuxer, options);
+}
diff --git a/src/functions/map.ts b/src/functions/map.ts
index 0bf708d..61e84d5 100644
--- a/src/functions/map.ts
+++ b/src/functions/map.ts
@@ -14,6 +14,7 @@ export function map<T, R>(
         writableObjectMode: true,
     },
 ): Transform {
+    // remove try catch
     return new Transform({
         ...options,
         async transform(chunk: T, encoding, callback) {
@@ -22,6 +23,7 @@ export function map<T, R>(
                 this.push(mapped);
                 callback();
             } catch (err) {
+                console.log("caught error", err.message);
                 callback(err);
             }
         },
diff --git a/src/index.ts b/src/index.ts
index 98b0a45..924b246 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -22,4 +22,5 @@ export {
     accumulator,
     accumulatorBy,
     compose,
+    demux,
 } from "./functions";
diff --git a/tests/demux.spec.ts b/tests/demux.spec.ts
new file mode 100644
index 0000000..cdc91d5
--- /dev/null
+++ b/tests/demux.spec.ts
@@ -0,0 +1,164 @@
+import test from "ava";
+import { expect } from "chai";
+import { demux, map } from "../src";
+import { Writable } from "stream";
+
+interface Test {
+    key: string;
+    val: number;
+}
+test.cb("should spread per key", t => {
+    t.plan(5);
+    const input = [
+        { key: "a", val: 1 },
+        { key: "b", val: 2 },
+        { key: "a", val: 3 },
+        { key: "c", val: 4 },
+    ];
+    const results = [
+        { key: "a", val: 2 },
+        { key: "b", val: 3 },
+        { key: "a", val: 4 },
+        { key: "c", val: 5 },
+    ];
+    const destinationStreamKeys = [];
+    let i = 0;
+    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();
+        },
+    });
+    const construct = (destKey: string) => {
+        destinationStreamKeys.push(destKey);
+        const dest = map((chunk: Test) => {
+            return {
+                ...chunk,
+                val: chunk.val + 1,
+            };
+        });
+
+        dest.pipe(sink);
+        return dest;
+    };
+
+    const demuxed = demux(construct, { key: "key" }, { objectMode: true });
+    demuxed.on("finish", () => {
+        expect(destinationStreamKeys).to.deep.equal(["a", "b", "c"]);
+        t.pass();
+        t.end();
+    });
+
+    input.forEach(event => demuxed.write(event));
+    demuxed.end();
+});
+
+test.cb("should spread per key using keyBy", t => {
+    t.plan(5);
+    const input = [
+        { key: "a", val: 1 },
+        { key: "b", val: 2 },
+        { key: "a", val: 3 },
+        { key: "c", val: 4 },
+    ];
+    const results = [
+        { key: "a", val: 2 },
+        { key: "b", val: 3 },
+        { key: "a", val: 4 },
+        { key: "c", 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();
+        },
+    });
+    const construct = (destKey: string) => {
+        destinationStreamKeys.push(destKey);
+        const dest = map((chunk: Test) => {
+            return {
+                ...chunk,
+                val: chunk.val + 1,
+            };
+        });
+
+        dest.pipe(sink);
+        return dest;
+    };
+
+    const demuxed = demux(
+        construct,
+        { keyBy: (chunk: any) => chunk.key },
+        { objectMode: true },
+    );
+    demuxed.on("finish", () => {
+        expect(destinationStreamKeys).to.deep.equal(["a", "b", "c"]);
+        t.pass();
+        t.end();
+    });
+
+    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();
+});
diff --git a/tslint.json b/tslint.json
index becd92c..03b2e44 100644
--- a/tslint.json
+++ b/tslint.json
@@ -9,6 +9,7 @@
         "no-implicit-dependencies": [true, "dev"],
         "prettier": [true, ".prettierrc"],
         "ordered-imports": false,
-        "interface-name": false
+        "interface-name": false,
+	"object-literal-sort-keys": false
     }
 }