jp: bounds-check the index in the at() function

pkg/jp/functions/at.go

@@ -2,6 +2,7 @@ package functions
 
 import (
 	"errors"
+	"fmt"
 )
 
 func jpfAt(arguments []any) (any, error) {
@@ -10,6 +11,10 @@ func jpfAt(arguments []any) (any, error) {
 	} else if index, ok := arguments[1].(float64); !ok {
 		return nil, errors.New("invalid type, second argument must be an int")
 	} else {
-		return slice[int(index)], nil
+		i := int(index)
+		if i < 0 || i >= len(slice) {
+			return nil, fmt.Errorf("index out of range: %d (array length %d)", i, len(slice))
+		}
+		return slice[i], nil
 	}
 }

pkg/jp/functions/at_test.go

@@ -0,0 +1,47 @@
+package functions
+
+import (
+	"testing"
+)
+
+func Test_jpfAt(t *testing.T) {
+	tests := []struct {
+		name    string
+		args    []any
+		want    any
+		wantErr bool
+	}{{
+		name: "valid index",
+		args: []any{[]any{"a", "b"}, float64(1)},
+		want: "b",
+	}, {
+		name: "first element",
+		args: []any{[]any{"a", "b"}, float64(0)},
+		want: "a",
+	}, {
+		name:    "out of range",
+		args:    []any{[]any{"a", "b"}, float64(999)},
+		wantErr: true,
+	}, {
+		name:    "negative index",
+		args:    []any{[]any{"a", "b"}, float64(-1)},
+		wantErr: true,
+	}}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := jpfAt(tt.args)
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("jpfAt() error = %v, wantErr %v", err, tt.wantErr)
+			}
+			if tt.wantErr {
+				if got != nil {
+					t.Fatalf("jpfAt() returned a value %v on error, want nil", got)
+				}
+				return
+			}
+			if got != tt.want {
+				t.Fatalf("jpfAt() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}