InjectTestAction.java

public class InjectTestAction extends AnAction {

  @Override
  public void actionPerformed(AnActionEvent e) {
    Project project = getEventProject(e);
    Editor editor = e.getData(PlatformDataKeys.EDITOR);
    PsiFile file = e.getData(PlatformDataKeys.PSI_FILE);
    Caret caret = e.getData(PlatformDataKeys.CARET);

    JSParameterList injectParameterList = findInjectParameterList(file);

    // Subtract 1 because it ends with ).
    int paramListOffset = injectParameterList.getTextRange().getEndOffset() - 1;

    // Surround injectable with underscores.
    String injectableWithUnderscores = "_" + caret.getSelectedText() + "_";

    // Add comma if this is the second+ parameter.
    boolean hasParams = injectParameterList.getChildren().length > 0;
    String injectParam = hasParams ? ", " + injectableWithUnderscores : injectableWithUnderscores;

    Document document = editor.getDocument();
    CommandProcessor.getInstance().executeCommand(project, () -> {
      ApplicationManager.getApplication().runWriteAction(() -> {
        // Assign the variable <selection> = _<selection>_;
        document.insertString(caret.getSelectionEnd(), " = " + injectableWithUnderscores + ";");

        // Add injectable at the end of the parameter list.
        document.insertString(paramListOffset, injectParam);
      });
    }, "inject test", null);
  }

  JSParameterList findInjectParameterList(PsiFile file) {...}
}