A simple REPL(read-eval-print loop) implementation is provided in the editor which can be open from the bottom panel GodotJS.
A dropdown list will be shown if auto-complete is available for the input partial expression. Press Up/Down to select one, and Tab to confirm the selection.
Note
So far, REPL runs on the javascript runtime the ScriptLanguage using.
And, the auto-complete feature may have side effects because the partial input is evaluated to get available candidates.