lastword.js

  const findLastWord = (editor) => {
      console.log(editor.selection);
      Transforms.move(editor, { unit: 'word', reverse: true })
      console.log(editor.selection);
      const selected = Editor.string(editor, editor.selection);
      console.log(selected);
      Transforms.move(editor, {unit: 'word'});
      editor.insertText(' ');
  };
// ...
  return (
    <Slate
      editor={editor}
      value={value}
      onChange={change => {
        onChange(change);
        setValue(change);
      }}
      onBlur={onBlur}
    >
      <Toolbar />
      <Editable
        renderElement={renderElement}
        renderLeaf={renderLeaf}
        onKeyDown={e => {
            e.preventDefault();
            const lastWord = findLastWord(editor);
        }}
      />
    </Slate>
  );