feat: add KeysBy function

maputil/map.go

@@ -34,6 +34,18 @@ func Values[K comparable, V any](m map[K]V) []V {
 	return values
 }
 
+// KeysBy creates a slice whose element is the result of function mapper invoked by every map's key.
+// todo:
+func KeysBy[K comparable, V any, T any](m map[K]V, mapper func(item K) T) []T {
+	keys := make([]T, 0, len(m))
+
+	for k := range m {
+		keys = append(keys, mapper(k))
+	}
+
+	return keys
+}
+
 // Merge maps, next key will overwrite previous key.
 // Play: https://go.dev/play/p/H95LENF1uB-
 func Merge[K comparable, V any](maps ...map[K]V) map[K]V {

maputil/map_test.go

@@ -41,6 +41,24 @@ func TestValues(t *testing.T) {
 	assert.Equal([]string{"a", "a", "b", "c", "d"}, values)
 }
 
+func TestKeysBy(t *testing.T) {
+	assert := internal.NewAssert(t, "TestKeysBy")
+
+	m := map[int]string{
+		1: "a",
+		2: "a",
+		3: "b",
+	}
+
+	keys := KeysBy(m, func(n int) int {
+		return n + 1
+	})
+
+	sort.Ints(keys)
+
+	assert.Equal([]int{2, 3, 4}, keys)
+}
+
 func TestMerge(t *testing.T) {
 	assert := internal.NewAssert(t, "TestMerge")