How to set DOM element as the first child?

var eElement; // some E DOM instance
var newFirstElement; //element which should be first in E

eElement.insertBefore(newFirstElement, eElement.firstChild);

2018 version - prepend

parent.prepend(newChild)  // [newChild, child1, child2]

This is modern JS! It is more readable than previous options. It is currently available in Chrome, FF, and Opera.

The equivalent for adding to the end is append, replacing the old appendChild

parent.append(newChild)  // [child1, child2, newChild]

Advanced usage

1. You can pass multiple values (or use spread operator ...).
2. Any string value will be added as a text element.

Examples:

parent.prepend(newChild, "foo")   // [newChild, "foo", child1, child2]

const list = ["bar", newChild]
parent.append(...list, "fizz")    // [child1, child2, "bar", newChild, "fizz"]

Related DOM methods

1. Read More - child.before and child.after
2. Read More - child.replaceWith

Mozilla Documentation

Can I Use

2017 version

You can use

targetElement.insertAdjacentElement('afterbegin', newFirstElement)

From MDN :

The insertAdjacentElement() method inserts a given element node at a given position relative to the element it is invoked upon.

position
A DOMString representing the position relative to the element; must be one of the following strings:
beforebegin: Before the element itself.
afterbegin: Just inside the element, before its first child.
beforeend: Just inside the element, after its last child.
afterend: After the element itself.

element
The element to be inserted into the tree.

Also in the family of insertAdjacent there is the sibling methods:

element.insertAdjacentHTML('afterbegin','htmlText') for inject html string directly, like innerHTML but without overide everything , so you can jump oppressive process of document.createElement and even build whole componet with string manipulation process

element.insertAdjacentText for inject sanitize string into element . no more encode/decode

You can implement it directly i all your window html elements.
Like this :

HTMLElement.prototype.appendFirst = function(childNode) {
    if (this.firstChild) {
        this.insertBefore(childNode, this.firstChild);
    }
    else {
        this.appendChild(childNode);
    }
};

Accepted answer refactored into a function:

function prependChild(parentEle, newFirstChildEle) {
    parentEle.insertBefore(newFirstChildEle, parentEle.firstChild)
}