feat: expose default equal function (#115)

src/index.spec.ts

@@ -17,6 +17,7 @@ import {
   SubscriberObject,
   asWritable,
   asReadable,
+  equal,
   batch,
   computed,
   derived,
@@ -659,6 +660,39 @@ describe('stores', () => {
       fixture.destroy();
       expect(componentInstance.store.hasListeners).toBe(false);
     });
+
+    it('equal should work as designed', () => {
+      // returns false for objects and functions:
+      const obj = {};
+      const fn = () => {};
+      expect(equal(obj, obj)).toBe(false);
+      expect(equal(fn, fn)).toBe(false);
+      expect(equal(obj, fn)).toBe(false);
+      expect(equal(fn, obj)).toBe(false);
+      expect(equal(obj, {})).toBe(false);
+      expect(equal(fn, () => {})).toBe(false);
+      // works as Object.is for other values:
+      expect(equal(true, true)).toBe(true);
+      expect(equal(false, false)).toBe(true);
+      expect(equal(true, false)).toBe(false);
+      expect(equal(false, true)).toBe(false);
+      expect(equal(0, 0)).toBe(true);
+      expect(equal(1, 1)).toBe(true);
+      expect(equal(NaN, NaN)).toBe(true);
+      expect(equal(NaN, null)).toBe(false);
+      expect(equal(null, NaN)).toBe(false);
+      expect(equal(Infinity, NaN)).toBe(false);
+      expect(equal(Infinity, Infinity)).toBe(true);
+      expect(equal(-Infinity, -Infinity)).toBe(true);
+      expect(equal(Infinity, -Infinity)).toBe(false);
+      expect(equal(-1, 1)).toBe(false);
+      expect(equal('a', 'b')).toBe(false);
+      expect(equal('a', 'a')).toBe(true);
+      expect(equal(null, undefined)).toBe(false);
+      expect(equal(undefined, null)).toBe(false);
+      expect(equal(undefined, undefined)).toBe(true);
+      expect(equal(null, null)).toBe(true);
+    });
   });
 
   describe('readable shorthand', () => {

src/index.ts

@@ -423,6 +423,19 @@ const createEqualCache = (valueIndex: number): Record<number, boolean> => ({
 
 const skipEqualInSet = Symbol();
 
+/**
+ * Default implementation of the equal function used by tansu when a store
+ * changes, to know if listeners need to be notified.
+ * Returns false if `a` is a function or an object, or if `a` and `b`
+ * are different according to `Object.is`. Otherwise, returns true.
+ *
+ * @param a - First value to compare.
+ * @param b - Second value to compare.
+ * @returns true if a and b are considered equal.
+ */
+export const equal = <T>(a: T, b: T): boolean =>
+  Object.is(a, b) && (!a || typeof a !== 'object') && typeof a !== 'function';
+
 /**
  * Base class that can be extended to easily create a custom {@link Readable} store.
  *
@@ -566,7 +579,7 @@ export abstract class Store<T> implements Readable<T> {
    * @returns true if a and b are considered different.
    */
   protected notEqual(a: T, b: T): boolean {
-    return !Object.is(a, b) || (a && typeof a === 'object') || typeof a === 'function';
+    return !equal(a, b);
   }
 
   /**