How about visibility: hidden;? Or opacity: 0;? So it’s still there & included in the layout, but invisible? I guess we’d need something to prevent clicks/focus on it as well? Or maybe we make it visible again on hover/focus? 
One relatively safe way would be to mimic the user interaction:
if(document.body.classList.contains("has-sidebar-page")){ // Sidebar is visible
document.querySelector(".btn-sidebar-toggle").click()
}