Func join (#499)

* Add join function

* Fix a typo

Author: TomWright

CHANGELOG.md

@@ -7,7 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet.
+### Added
+- `join` function to join array elements into a single string with a specified separator. [See docs](https://daseldocs.tomwright.me/functions/join).
 
 ## [v3.1.4] - 2025-12-18
 

execution/func.go

@@ -29,6 +29,7 @@ var (
 		FuncGet,
 		FuncContains,
 		FuncSum,
+		FuncJoin,
 	)
 )
 

execution/func_join.go

@@ -0,0 +1,60 @@
+package execution
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"github.com/tomwright/dasel/v3/model"
+)
+
+// FuncJoin is a function that joins the given data or args to a string.
+var FuncJoin = NewFunc(
+	"join",
+	func(ctx context.Context, data *model.Value, args model.Values) (*model.Value, error) {
+		separator, err := args[0].StringValue()
+		if err != nil {
+			return nil, fmt.Errorf("join expects a string separator as the first argument: %w", err)
+		}
+
+		var valuesToJoin []string
+
+		if len(args) == 2 && args[1].IsSlice() {
+			if err := args[1].RangeSlice(func(i int, value *model.Value) error {
+				strVal, err := value.StringValue()
+				if err != nil {
+					return fmt.Errorf("could not read string value of index %d: %w", i, err)
+				}
+				valuesToJoin = append(valuesToJoin, strVal)
+				return nil
+			}); err != nil {
+				return nil, err
+			}
+		} else if len(args) > 1 {
+			// Join the args
+			for i := 1; i < len(args); i++ {
+				strVal, err := args[i].StringValue()
+				if err != nil {
+					return nil, fmt.Errorf("could not read string value of argument index %d: %w", i, err)
+				}
+				valuesToJoin = append(valuesToJoin, strVal)
+			}
+		} else {
+			if err := data.RangeSlice(func(i int, value *model.Value) error {
+				strVal, err := value.StringValue()
+				if err != nil {
+					return fmt.Errorf("could not read string value of index %d: %w", i, err)
+				}
+				valuesToJoin = append(valuesToJoin, strVal)
+				return nil
+			}); err != nil {
+				return nil, err
+			}
+		}
+
+		joined := strings.Join(valuesToJoin, separator)
+
+		return model.NewStringValue(joined), nil
+	},
+	ValidateArgsMin(1),
+)

execution/func_join_test.go

@@ -0,0 +1,23 @@
+package execution_test
+
+import (
+	"testing"
+
+	"github.com/tomwright/dasel/v3/model"
+	_ "github.com/tomwright/dasel/v3/parsing/json"
+)
+
+func TestFuncJoin(t *testing.T) {
+	t.Run("chained input", testCase{
+		s:   `["a","b","c"].join(",")`,
+		out: model.NewStringValue("a,b,c"),
+	}.run)
+	t.Run("vararg input", testCase{
+		s:   `join(",", "a", "b", "c")`,
+		out: model.NewStringValue("a,b,c"),
+	}.run)
+	t.Run("array input", testCase{
+		s:   `join(",", ["a", "b", "c"])`,
+		out: model.NewStringValue("a,b,c"),
+	}.run)
+}