feat: adds old value as a second parameter to listeners

Author: rasulomaroff

package.json

@@ -1,6 +1,6 @@
 {
   "name": "react-rv",
-  "version": "0.2.5",
+  "version": "0.3.0",
   "description": "react-rv is a lightweight and efficient state management library for React that allows you to create reactive variables and subscribe to them with minimal overhead.",
   "files": [
     "dist"

src/rv.spec-d.ts

@@ -27,13 +27,15 @@ describe('rv function', () => {
 
     it('infers variable type in a subscription parameter', () => {
         const val = rv(2, {
-            on: v => {
+            on: (v, o) => {
                 expectTypeOf(v).toBeNumber()
+                expectTypeOf(o).toBeNumber()
             },
         })
 
-        val.on(v => {
+        val.on((v, o) => {
             expectTypeOf(v).toBeNumber()
+            expectTypeOf(o).toBeNumber()
         })
     })
 
@@ -63,13 +65,15 @@ describe('rv function', () => {
 
     it('infers variable type in a subscription parameter', () => {
         const val = rv.fn(() => 3, {
-            on: v => {
+            on: (v, o) => {
                 expectTypeOf(v).toBeNumber()
+                expectTypeOf(o).toBeNumber()
             },
         })
 
-        val.on(v => {
+        val.on((v, o) => {
             expectTypeOf(v).toBeNumber()
+            expectTypeOf(o).toBeNumber()
         })
     })
 

src/rv.spec.ts

@@ -49,7 +49,7 @@ describe('rv function', () => {
         val.on(listener)
 
         val(3)
-        expect(listener).toHaveBeenCalledExactlyOnceWith(3)
+        expect(listener).toHaveBeenCalledExactlyOnceWith(3, 2)
     })
 
     it('allows to pass the default listener', () => {
@@ -60,7 +60,7 @@ describe('rv function', () => {
         val.on(listener)
 
         val(3)
-        expect(listener).toHaveBeenCalledExactlyOnceWith(3)
+        expect(listener).toHaveBeenCalledExactlyOnceWith(3, 2)
     })
 
     it('does not call listeners if value does not change', () => {

src/rv.ts

@@ -6,6 +6,7 @@ const defaultEq = <T>(oldValue: T, newValue: T): boolean => oldValue === newValu
  * Creates a reactive variable (RV), allowing value retrieval, updates, and subscriptions.
  *
  * @template T The type of the stored value.
+ *
  * @param val The initial value of the reactive variable.
  * @param options Optional configuration for the reactive variable.
  *
@@ -22,7 +23,7 @@ const defaultEq = <T>(oldValue: T, newValue: T): boolean => oldValue === newValu
  *     eq: (oldValue, newValue) => newValue > oldValue && newValue >= 0,
  *
  *     // define callback that's going to be run on every change
- *     on: val => {}
+ *     on: (newValue, oldValue) => {}
  * })
  *
  * // alternatively, there's a handy function initializer
@@ -43,7 +44,7 @@ const defaultEq = <T>(oldValue: T, newValue: T): boolean => oldValue === newValu
  * })
  *
  * // you can also subscribe to value without using any hooks
- * const unsubscribe = positiveVar.on(newValue => console.log(newValue))
+ * const unsubscribe = positiveVar.on((newValue, oldValue) => console.log(newValue, oldValue))
  *
  * positiveVar(4) // logs: 4
  *
@@ -72,9 +73,10 @@ export function rv<T>(val: T, options?: RvInitOptions<T>): Rv<T> {
 
         if (eqfn(val, newValue)) return val
 
+        const oldValue = val
         val = newValue
 
-        listeners.forEach(cb => cb(newValue))
+        listeners.forEach(cb => cb(newValue, oldValue))
 
         return val
     }

src/types.ts

@@ -2,9 +2,11 @@
  * A function that listens to reactive variable updates.
  *
  * @template T The type of the value being listened to.
+ *
  * @param val The updated value of the reactive variable.
+ * @param old The old value of the reactive variable.
  */
-export type Listener<T> = (val: T) => void
+export type Listener<T> = (val: T, old: T) => void
 
 /**
  * A function that cleans up event listener.
@@ -15,6 +17,7 @@ export type CleanupFn = () => void
  * A function to compare old and new values for equality.
  *
  * @template T The type of values being compared.
+ *
  * @param oldValue The previous value.
  * @param newValue The new value to compare against.
  *
@@ -81,7 +84,7 @@ export type RvInitOptions<T> = {
      *
      * @param val The new value of the reactive variable.
      */
-    on?: (val: T) => void
+    on?: Listener<T>
 }
 
 /**
@@ -92,6 +95,7 @@ export interface RvInit {
      * Creates a reactive variable (RV), allowing value retrieval, updates, and subscriptions.
      *
      * @template T The type of the stored value.
+     *
      * @param val The initial value of the reactive variable.
      * @param options Optional configuration for the reactive variable.
      *
@@ -103,6 +107,7 @@ export interface RvInit {
      * The function is immediately executed to determine the initial value.
      *
      * @template T The type of the stored value.
+     *
      * @param init A function that returns the initial value.
      * @param options Optional configuration for equality comparison and event listeners.
      *