Cannot read property 'addEventListener' of null at /script.js:4:34 code example

Example 1: Error: TypeError: Cannot read property 'addEventListener' of null at downLoaded (file:///convert.js:10:38)

if(cInputValue.value != "")

Example 2: 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
}