app.js:5 Uncaught TypeError: Cannot read property 'addEventListener' of null code example

Example: Cannot read property 'addEventListener' of null

if (document.body.contains(document.getElementById('someid'))) {
    // do what you need
}

// or 

if (!document.visibilityState) {
    // do what you need
}

// or 

if (!document.getElementsByClassName('.someclass')){
    // do what you need
}