diff --git a/src/testdir/vim9.vim b/src/testdir/vim9.vim
new file mode 100644
index 0000000..0e0cd1e
--- /dev/null
+++ b/src/testdir/vim9.vim
@@ -0,0 +1,28 @@
+" Utility functions for testing vim9 script
+
+" Check that "lines" inside ":def" results in an "error" message.
+func CheckDefFailure(lines, error)
+  call writefile(['def Func()'] + a:lines + ['enddef'], 'Xdef')
+  call assert_fails('so Xdef', a:error, a:lines)
+  call delete('Xdef')
+endfunc
+
+def CheckScriptFailure(lines: list<string>, error: string)
+  writefile(lines, 'Xdef')
+  assert_fails('so Xdef', error, lines)
+  delete('Xdef')
+enddef
+
+def CheckScriptSuccess(lines: list<string>)
+  writefile(lines, 'Xdef')
+  so Xdef
+  delete('Xdef')
+enddef
+
+" Check that "line" inside ":def" results in an "error" message when executed.
+func CheckDefExecFailure(line, error)
+  call writefile(['def! Func()', a:line, 'enddef'], 'Xdef')
+  so Xdef
+  call assert_fails('call Func()', a:error, a:line)
+  call delete('Xdef')
+endfunc