test: test user function

types/test.ts

@@ -105,3 +105,50 @@ const g = {
 deepmerge(f, g);
 // $ExpectType { key1: string[]; key2: { key2key1: { key2key1key1: () => string; key2key1key2: boolean; key2key1key3: string[]; }; }; key3: { key3key1: { key3key1key1: string; key3key1key2: boolean; key3key1key3: string[]; }; key3key2: (string | boolean)[]; }; }
 deepmerge.all([f, g]);
+
+// $ExpectType { foo: any; baz: any; garply: any; grault: any; }
+deepmerge(a, b, {
+	isMergeableObject: (obj: object) => {
+		return false;
+	},
+});
+// $ExpectType { foo: any; baz: any; garply: any; grault: any; }
+deepmerge.all([a, b], {
+	isMergeableObject: (obj: object) => {
+		return false;
+	},
+});
+
+const h = {
+	baz: {
+		quuz: [5, 6],
+	},
+};
+
+// $ExpectType { foo: string; baz: { quuz: any; }; garply: number; }
+deepmerge(a, h, {
+	arrayMerge: (target: unknown[], source: unknown[]) =>
+		target.length + source.length,
+});
+// $ExpectType { foo: string; baz: { quuz: any; }; garply: number; }
+deepmerge.all([a, h], {
+	arrayMerge: (target: unknown[], source: unknown[]) =>
+		target.length + source.length,
+});
+
+// $ExpectType { foo: any; baz: any; garply: any; }
+deepmerge(a, h, {
+	customMerge: (key: string) => {
+			if (key === 'foo') {
+				return (target: string, source: string) => target + source;
+			}
+    }
+});
+// $ExpectType { foo: any; baz: any; garply: any; }
+deepmerge.all([a, h], {
+	customMerge: (key: string) => {
+			if (key === 'foo') {
+				return (target: string, source: string) => target + source;
+			}
+    }
+});