Add keys func (#533)

Author: TomWright

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet.
+### Added
+
+- `keys` func that returns the keys or indices of a node. [See docs](https://daseldocs.tomwright.me/functions/keys).
 
 ## [v3.3.2] - 2026-03-18
 

execution/func.go

@@ -31,6 +31,7 @@ var (
 		FuncSum,
 		FuncJoin,
 		FuncReplace,
+		FuncKeys,
 	)
 )
 

execution/func_keys.go

@@ -0,0 +1,44 @@
+package execution
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/tomwright/dasel/v3/model"
+)
+
+// FuncKeys returns the keys of a map or the indices of a slice.
+var FuncKeys = NewFunc(
+	"keys",
+	func(ctx context.Context, data *model.Value, args model.Values) (*model.Value, error) {
+		switch data.Type() {
+		case model.TypeMap:
+			keys, err := data.MapKeys()
+			if err != nil {
+				return nil, err
+			}
+			res := model.NewSliceValue()
+			for _, key := range keys {
+				if err := res.Append(model.NewStringValue(key)); err != nil {
+					return nil, err
+				}
+			}
+			return res, nil
+		case model.TypeSlice:
+			len, err := data.SliceLen()
+			if err != nil {
+				return nil, err
+			}
+			res := model.NewSliceValue()
+			for i := 0; i < len; i++ {
+				if err := res.Append(model.NewIntValue(int64(i))); err != nil {
+					return nil, err
+				}
+			}
+			return res, nil
+		default:
+			return nil, fmt.Errorf("keys can only be used on maps and slices")
+		}
+	},
+	ValidateArgsExactly(0),
+)

execution/func_keys_test.go

@@ -0,0 +1,35 @@
+package execution_test
+
+import (
+	"testing"
+
+	"github.com/tomwright/dasel/v3/model"
+	_ "github.com/tomwright/dasel/v3/parsing/json"
+)
+
+func TestFuncKeys(t *testing.T) {
+	t.Run("returns slice indices", testCase{
+		s: `[1,2,3,4,5].keys()`,
+		outFn: func() *model.Value {
+			r := model.NewSliceValue()
+			for i := 0; i < 5; i++ {
+				if err := r.Append(model.NewIntValue(int64(i))); err != nil {
+					panic(err)
+				}
+			}
+			return r
+		},
+	}.run)
+	t.Run("returns map keys", testCase{
+		s: `{"a": 3, "b": 4, "c": 5}.keys()`,
+		outFn: func() *model.Value {
+			r := model.NewSliceValue()
+			for _, key := range []string{"a", "b", "c"} {
+				if err := r.Append(model.NewStringValue(key)); err != nil {
+					panic(err)
+				}
+			}
+			return r
+		},
+	}.run)
+}