How to move all HTML element children to another parent using JavaScript?

Imagine:

<div id="old-parent">
    <span>Foo</span>
    <b>Bar</b>
    Hello World
</div>
<div id="new-parent"></div>

What JavaScript can be written to move all the child nodes (both elements, and text nodes) from old-parent to new-parent without jQuery?

I don't care about whitespace between nodes, though I expect to catch the stray Hello World, they would need to be migrated as-is too.

EDIT

To be clear, I want to end up with:

<div id="old-parent"></div>
<div id="new-parent">
    <span>Foo</span>
    <b>Bar</b>
    Hello World
</div>

The answer of the question from which this is a proposed duplicate would result in:

<div id="new-parent">
    <div id="old-parent">
        <span>Foo</span>
        <b>Bar</b>
        Hello World
    </div>
</div>

Basically, you want to loop through each direct descendent of the old-parent node, and move it to the new parent. Any children of a direct descendent will get moved with it.

var newParent = document.getElementById('new-parent');
var oldParent = document.getElementById('old-parent');

function move() {
  while (oldParent.childNodes.length > 0) {
    newParent.appendChild(oldParent.childNodes[0]);
  }
}

#old-parent {
  background-color: red;
}

#new-parent {
  background-color: green;
}

<div id="old-parent">
  <span>Foo</span>
  <b>Bar</b> Hello World
</div>
<div id="new-parent"></div>
<br>
<button onclick="move()" id="button">Move childs</button>

External link

Modern way:

newParent.append(...oldParent.childNodes);

1. .append is the replacement for .appendChild. The main difference is that it accepts multiple nodes at once and even plain strings, like .append('hello!')
2. oldParent.childNodes is iterable so it can be spread with ... to become multiple parameters of .append()

It's supported by every browser except IE.

Here's a simple function:

function setParent(el, newParent)
{
    newParent.appendChild(el);
}

el's childNodes are the elements to be moved, newParent is the element el will be moved to, so you would execute the function like:

var l = document.getElementById('old-parent').childNodes.length;
var a = document.getElementById('old-parent');
var b = document.getElementById('new-parent');
for (var i = l; i >= 0; i--)
{
    setParent(a.childNodes[0], b);
}

Here is the Demo

If you not use - in id's names then you can do this

oldParent.id='xxx';
newParent.id='oldParent';
xxx.id='newParent';
oldParent.parentNode.insertBefore(oldParent,newParent);

#newParent { color: red }

<div id="oldParent">
    <span>Foo</span>
    <b>Bar</b>
    Hello World
</div>
<div id="newParent"></div>

This answer only really works if you don't need to do anything other than transferring the inner code (innerHTML) from one to the other:

// Define old parent
var oldParent = document.getElementById('old-parent');

// Define new parent
var newParent = document.getElementById('new-parent');

// Basically takes the inner code of the old, and places it into the new one
newParent.innerHTML = oldParent.innerHTML;

// Delete / Clear the innerHTML / code of the old Parent
oldParent.innerHTML = '';

Hope this helps!