diff --git a/package.json b/package.json
index 38dc4c5..8009f9f 100644
--- a/package.json
+++ b/package.json
@@ -31,6 +31,7 @@
     "devDependencies": {
         "@types/chai": "^4.1.7",
         "@types/node": "^10.12.10",
+        "@types/typescript": "^2.0.0",
         "ava": "^1.0.0-rc.2",
         "chai": "^4.2.0",
         "mhysa": "./",
diff --git a/src/functions/definitions.ts b/src/functions/definitions.ts
index f18659a..8abb2ea 100644
--- a/src/functions/definitions.ts
+++ b/src/functions/definitions.ts
@@ -22,18 +22,3 @@ export interface JsonParseOptions {
     pretty: boolean;
 }
 
-export interface IBatchParams {
-    batchSize?: number;
-    maxBatchAge?: number;
-}
-
-export interface IRateParams {
-    targetRate?: number;
-    period?: number;
-}
-
-export interface IParallelMapParams<T, R> {
-    mapper: (data: T) => R;
-    parallel?: number;
-    sleepTime?: number;
-}
diff --git a/src/functions/functions.spec.ts b/src/functions/functions.spec.ts
index 50e5916..02e5303 100644
--- a/src/functions/functions.spec.ts
+++ b/src/functions/functions.spec.ts
@@ -25,7 +25,7 @@ import {
     rate,
     parallelMap,
 } from ".";
-import { SerializationFormats } from "./definitions";
+import { sleep } from "../helpers";
 
 test.cb("fromArray() streams array elements in flowing mode", t => {
     t.plan(3);
@@ -683,7 +683,7 @@ test.cb("parse() parses the streamed elements as JSON", t => {
     const expectedElements = ["abc", {}, []];
     let i = 0;
     source
-        .pipe(parse(SerializationFormats.utf8))
+        .pipe(parse())
         .on("data", part => {
             expect(part).to.deep.equal(expectedElements[i]);
             t.pass();
@@ -702,7 +702,7 @@ test.cb("parse() emits errors on invalid JSON", t => {
     t.plan(2);
     const source = new Readable({ objectMode: true });
     source
-        .pipe(parse(SerializationFormats.utf8))
+        .pipe(parse())
         .resume()
         .on("error", () => t.pass())
         .on("end", t.end);
@@ -1297,7 +1297,7 @@ test.cb("rate() sends data at desired rate", t => {
 });
 
 test.cb("parallel() parallel mapping", t => {
-    t.plan(5);
+    t.plan(6);
     const source = new Readable({ objectMode: true });
     const expectedElements = [
         "a_processed",
@@ -1305,22 +1305,48 @@ test.cb("parallel() parallel mapping", t => {
         "c_processed",
         "d_processed",
         "e_processed",
+        "f_processed",
     ];
-    const orderedResults: string[] = [];
+    const orderedResults: Array<{ output: string; processed: number }> = [];
     // Record start / end times of each process and then compare to figure out # of processes ocurring and order
     source
-        .pipe(parallelMap(data => data + "_processed"))
+        .pipe(
+            parallelMap(async (data: any) => {
+                const c = data + "_processed";
+                await sleep(500);
+                orderedResults.push({
+                    output: c,
+                    processed: performance.now(),
+                });
+                return c;
+            }, 2),
+        )
         .on("data", (element: string) => {
             t.true(expectedElements.includes(element));
-            orderedResults.push(element);
         })
         .on("error", t.end)
-        .on("end", () => {
-            expect(orderedResults[0]).to.equal("a_processed");
-            expect(orderedResults[1]).to.equal("b_processed");
-            expect(orderedResults[2]).to.equal("d_processed");
-            expect(orderedResults[3]).to.equal("c_processed");
-            expect(orderedResults[4]).to.equal("e_processed");
+        .on("end", async () => {
+            expect(orderedResults[0].processed).to.be.lessThan(
+                orderedResults[1].processed + 500,
+            );
+            expect(orderedResults[2].processed).to.be.lessThan(
+                orderedResults[3].processed + 500,
+            );
+            expect(orderedResults[4].processed).to.be.lessThan(
+                orderedResults[5].processed + 500,
+            );
+            expect(orderedResults[2].processed).to.be.greaterThan(
+                orderedResults[0].processed + 500,
+            );
+            expect(orderedResults[3].processed).to.be.greaterThan(
+                orderedResults[1].processed + 500,
+            );
+            expect(orderedResults[4].processed).to.be.greaterThan(
+                orderedResults[2].processed + 500,
+            );
+            expect(orderedResults[5].processed).to.be.greaterThan(
+                orderedResults[3].processed + 500,
+            );
             t.end();
         });
 
@@ -1329,5 +1355,6 @@ test.cb("parallel() parallel mapping", t => {
     source.push("c");
     source.push("d");
     source.push("e");
+    source.push("f");
     source.push(null);
 });
diff --git a/src/functions/index.ts b/src/functions/index.ts
index 1ad0f10..9085930 100644
--- a/src/functions/index.ts
+++ b/src/functions/index.ts
@@ -7,9 +7,6 @@ import {
     TransformOptions,
     WithEncoding,
     JsonParseOptions,
-    IBatchParams,
-    IRateParams,
-    IParallelMapParams,
 } from "./definitions";
 
 /**
@@ -205,14 +202,10 @@ export function last<T>(readable: Readable): Promise<T | null> {
 
 /**
  * Stores chunks of data internally in array and batches when batchSize is reached.
- *
- * @param batchSize? Size of the batches, defaults to 1000.
+ * @param batchSize Size of the batches, defaults to 1000.
  * @param maxBatchAge? Max lifetime of a batch, defaults to 500
  */
-export function batch({
-    batchSize,
-    maxBatchAge,
-}: IBatchParams): NodeJS.ReadWriteStream {
+export function batch(batchSize: number, maxBatchAge?: number): NodeJS.ReadWriteStream {
     return baseFunctions.batch(batchSize, maxBatchAge);
 }
 
@@ -225,13 +218,11 @@ export function unbatch(): NodeJS.ReadWriteStream {
 
 /**
  * Limits date of data transferred into stream.
+ * @param options?
  * @param targetRate? Desired rate in ms
  * @param period? Period to sleep for when rate is above or equal to targetRate
  */
-export function rate({
-    targetRate,
-    period,
-}: IRateParams): NodeJS.ReadWriteStream {
+export function rate(targetRate?: number, period?: number): NodeJS.ReadWriteStream {
     return baseFunctions.rate(targetRate, period);
 }
 
@@ -241,10 +232,10 @@ export function rate({
  * @param func Function to execute on each data chunk
  * @param pause Amount of time to pause processing when max number of parallel processes are executing.
  */
-export function parallelMap<T, R>({
-    mapper,
-    parallel,
-    sleepTime,
-}: IParallelMapParams<T, R>) {
+export function parallelMap<T, R>(
+    mapper: (chunk: T) => R,
+    parallel?: number,
+    sleepTime?: number,
+) {
     return baseFunctions.parallelMap(mapper, parallel, sleepTime);
 }
diff --git a/yarn.lock b/yarn.lock
index 082e22d..dfb9135 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -335,6 +335,13 @@
   resolved "https://registry.yarnpkg.com/@types/node/-/node-10.12.10.tgz#4fa76e6598b7de3f0cb6ec3abacc4f59e5b3a2ce"
   integrity sha512-8xZEYckCbUVgK8Eg7lf5Iy4COKJ5uXlnIOnePN0WUwSQggy9tolM+tDJf7wMOnT/JT/W9xDYIaYggt3mRV2O5w==
 
+"@types/typescript@^2.0.0":
+  version "2.0.0"
+  resolved "https://registry.yarnpkg.com/@types/typescript/-/typescript-2.0.0.tgz#c433539c98bae28682b307eaa7a0fd2115b83c28"
+  integrity sha1-xDNTnJi64oaCswfqp6D9IRW4PCg=
+  dependencies:
+    typescript "*"
+
 abbrev@1:
   version "1.1.1"
   resolved "https://registry.yarnpkg.com/abbrev/-/abbrev-1.1.1.tgz#f8f2c887ad10bf67f634f005b6987fed3179aac8"
@@ -3205,6 +3212,11 @@ type-detect@^4.0.0, type-detect@^4.0.5:
   resolved "https://registry.yarnpkg.com/type-detect/-/type-detect-4.0.8.tgz#7646fb5f18871cfbb7749e69bd39a6388eb7450c"
   integrity sha512-0fr/mIH1dlO+x7TlcMy+bIDqKPsw/70tVyeHW787goQjhmqaZe10uwLujubK9q9Lg6Fiho1KUKDYz0Z7k7g5/g==
 
+typescript@*:
+  version "3.5.2"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.5.2.tgz#a09e1dc69bc9551cadf17dba10ee42cf55e5d56c"
+  integrity sha512-7KxJovlYhTX5RaRbUdkAXN1KUZ8PwWlTzQdHV6xNqvuFOs7+WBo10TQUqT19Q/Jz2hk5v9TQDIhyLhhJY4p5AA==
+
 typescript@^3.1.6:
   version "3.1.6"
   resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.1.6.tgz#b6543a83cfc8c2befb3f4c8fba6896f5b0c9be68"