changeDocument.java

public class InjectTestAction extends AnAction {

  @Override
  public void actionPerformed(AnActionEvent e) {
    PsiFile file = e.getData(PlatformDataKeys.PSI_FILE);
    Caret caret = e.getData(PlatformDataKeys.CARET);
    Editor editor = e.getData(PlatformDataKeys.EDITOR);
    
    JSParameterList injectParameterList = findInjectParameterList(file);
    
    // Subtract 1 because it ends with ).
    int paramListOffset = injectParameterList.getTextRange().getEndOffset() - 1;
 
    // Surround injectable with underscores.
    String injectableWithUnderscores = "_" + caret.getSelectedText() + "_";

    Document document = editor.getDocument();

    // Assign the variable <selection> = _<selection>_;
    document.insertString(caret.getSelectionEnd(),  " = " + injectableWithUnderscores + ";");

    // Add the dependency parameter list.
    document.insertString(paramListOffset, injectableWithUnderscores);
  }
  
  JSParameterList findInjectParameterList(...) { ... }
}