Question

As you can see I am still a novice in javascript
Why is it so that you can append a Textnode only once? When you add it again somewhere else the first one disappears

I do not need a solution to a problem I was just curious what is causing this behavior.
Example where the textnode is only added to the last element of an array:

function hideAdd(){
    var hide = document.createTextNode('Afbeelding verbergen');
    var afb = collectionToArray(document.getElementsByTagName('img'));  
    afb.pop();
    var divs = [];
    for (i=0; i < afb.length; i++){
        divs.push(afb[i].parentNode);

    }
    console.log(divs);
    for ( i = 0; i < divs.length;i++){
        divs[i].appendChild(hide);      
    }
}

This is where you use an unique textnode so it works:

function hideAdd(){
    var hide = []
    var afb = collectionToArray(document.getElementsByTagName('img'));  
    afb.pop();
    var divs = [];
    for (i=0; i < afb.length; i++){
        divs.push(afb[i].parentNode);
        hide[i] = document.createTextNode('Afbeelding verbergen');

    }
    console.log(divs);
    for ( i = 0; i < divs.length;i++){
        divs[i].appendChild(hide[i]);       
    }
}

Answer 1

Short answer is the DOM is a tree, not a network. Each node can have only one parent. If you could add a node in more than one location, it would have more than one parent.