diff --git a/packages/reactive-core/src/autorun.ts b/packages/reactive-core/src/autorun.ts
index f162052..2b7e93f 100644
--- a/packages/reactive-core/src/autorun.ts
+++ b/packages/reactive-core/src/autorun.ts
@@ -3,22 +3,20 @@ import { Reaction } from "./reaction";
 
 export function autorun<T>(
   func: () => T extends Promise<void> ? never : T extends void ? T : never,
-  extraOptions?: { fireImmediately?: boolean; name?: string }
+  extraOptions?: { name?: string }
 ): Reaction {
-  const options = Object.assign({ fireImmediately: true, name: "unnamed" }, extraOptions);
+  const options = { name: "unnamed", fireImmediately: true, ...extraOptions };
   const reaction = new Reaction(func, options);
-  if (options.fireImmediately) {
-    reaction.trigger();
-  }
+
   return reaction;
 }
 
 export function autorunAsync<T>(
   func: (reactive: T) => Promise<void>,
   reactiveObject: T,
-  extraOptions?: { fireImmediately?: boolean; name?: string }
+  extraOptions?: { name?: string }
 ): Reaction {
-  const options = Object.assign({ fireImmediately: true, name: "unnamed" }, extraOptions);
+  const options = { name: "unnamed", fireImmediately: true, ...extraOptions };
   const reaction = new Reaction(() => {
     func(reactiveObject); // TODO: error handling
   }, options);
diff --git a/packages/reactive-core/src/reaction.ts b/packages/reactive-core/src/reaction.ts
index 2202763..5fe1f65 100644
--- a/packages/reactive-core/src/reaction.ts
+++ b/packages/reactive-core/src/reaction.ts
@@ -2,8 +2,20 @@ import { Observer } from "./observer";
 
 let runningReactions: Reaction[] = [];
 export class Reaction extends Observer {
-  constructor(private func: () => void | Promise<void>, private options: { name: string }) {
+  private isInitial = true;
+
+  constructor(
+    private func: () => void | Promise<void>,
+    private options: { fireImmediately: boolean; name: string },
+    private effect?: () => void | Promise<void>
+  ) {
     super(() => this._trigger());
+
+    if (!effect && !this.options.fireImmediately) {
+      throw new Error("if no effect function passed, should always fireImmediately");
+    }
+    // fire reaction
+    this.reaction();
   }
 
   private reaction = () => {
@@ -14,6 +26,11 @@ export class Reaction extends Observer {
     } finally {
       runningReactions.pop();
     }
+
+    if (this.effect && (!this.isInitial || this.options.fireImmediately)) {
+      this.effect();
+    }
+    this.isInitial = false;
   };
 
   private _trigger() {
@@ -33,3 +50,13 @@ export function hasRunningReaction() {
 export function runningReaction() {
   return runningReactions.length ? runningReactions[runningReactions.length - 1] : undefined;
 }
+
+export function reaction(
+  func: () => any | Promise<any>,
+  effect: () => void | Promise<void>,
+  options?: { fireImmediately?: boolean; name?: string }
+) {
+  const newOptions = { name: "unnamed", fireImmediately: true, ...options };
+  const r = new Reaction(func, newOptions, effect);
+  return r;
+}
diff --git a/packages/reactive-core/test/autorun.test.ts b/packages/reactive-core/test/autorun.test.ts
index 6354db8..2b6cce1 100644
--- a/packages/reactive-core/test/autorun.test.ts
+++ b/packages/reactive-core/test/autorun.test.ts
@@ -1,4 +1,4 @@
-import { runInAction } from "@reactivedata/reactive";
+import { reaction, runInAction } from "@reactivedata/reactive";
 import { autorun, autorunAsync } from "../src/autorun";
 import { reactive } from "../src/observable";
 import { Observer } from "../src/observer";
@@ -592,15 +592,41 @@ describe("autorun", () => {
 describe("options", () => {
   describe("fireImmediately", () => {
     it("should not run the passed function, if set to true", () => {
-      const fnSpy = jest.fn(() => {});
-      autorun(fnSpy, { fireImmediately: false });
-      expect(fnSpy).toBeCalledTimes(0);
+      const nums = reactive({ num1: 0, num2: 1, num3: 2 });
+
+      const fnSpyTrigger = jest.fn(() => {
+        let x = nums.num1;
+      });
+      const fnSpyEffect = jest.fn(() => {
+        let x = nums.num1;
+      });
+      reaction(fnSpyTrigger, fnSpyEffect, { fireImmediately: false });
+      expect(fnSpyTrigger).toBeCalledTimes(1);
+      expect(fnSpyEffect).toBeCalledTimes(0);
+
+      nums.num1++;
+
+      expect(fnSpyTrigger).toBeCalledTimes(2);
+      expect(fnSpyEffect).toBeCalledTimes(1);
     });
 
     it("should default to true", () => {
-      const fnSpy = jest.fn(() => {});
-      autorun(fnSpy);
-      expect(fnSpy).toHaveBeenCalledTimes(1);
+      const nums = reactive({ num1: 0, num2: 1, num3: 2 });
+
+      const fnSpyTrigger = jest.fn(() => {
+        let x = nums.num1;
+      });
+      const fnSpyEffect = jest.fn(() => {
+        let x = nums.num1;
+      });
+      reaction(fnSpyTrigger, fnSpyEffect);
+      expect(fnSpyTrigger).toBeCalledTimes(1);
+      expect(fnSpyEffect).toBeCalledTimes(1);
+
+      nums.num1++;
+
+      expect(fnSpyTrigger).toBeCalledTimes(2);
+      expect(fnSpyEffect).toBeCalledTimes(2);
     });
   });