Wrapping a set of DOM elements using JavaScript

Posted below are a pure JavaScript version of jQuery’s wrap and wrapAll methods. I can’t guarantee they work exactly as they do in jQuery, but they do in fact work very similarly and should be able to accomplish the same tasks. They work with either a single HTMLElement or an array of them. I haven’t tested to confirm, but they should both work in all modern browsers (and older ones to a certain extent).

Unlike the selected answer, these methods maintain the correct HTML structure by using insertBefore as well as appendChild.

wrap:

// Wrap an HTMLElement around each element in an HTMLElement array.
HTMLElement.prototype.wrap = function(elms) {
    // Convert `elms` to an array, if necessary.
    if (!elms.length) elms = [elms];

    // Loops backwards to prevent having to clone the wrapper on the
    // first element (see `child` below).
    for (var i = elms.length - 1; i >= 0; i--) {
        var child = (i > 0) ? this.cloneNode(true) : this;
        var el    = elms[i];

        // Cache the current parent and sibling.
        var parent  = el.parentNode;
        var sibling = el.nextSibling;

        // Wrap the element (is automatically removed from its current
        // parent).
        child.appendChild(el);

        // If the element had a sibling, insert the wrapper before
        // the sibling to maintain the HTML structure; otherwise, just
        // append it to the parent.
        if (sibling) {
            parent.insertBefore(child, sibling);
        } else {
            parent.appendChild(child);
        }
    }
};

See a working demo on jsFiddle.

wrapAll:

// Wrap an HTMLElement around another HTMLElement or an array of them.
HTMLElement.prototype.wrapAll = function(elms) {
    var el = elms.length ? elms[0] : elms;

    // Cache the current parent and sibling of the first element.
    var parent  = el.parentNode;
    var sibling = el.nextSibling;

    // Wrap the first element (is automatically removed from its
    // current parent).
    this.appendChild(el);

    // Wrap all other elements (if applicable). Each element is
    // automatically removed from its current parent and from the elms
    // array.
    while (elms.length) {
        this.appendChild(elms[0]);
    }

    // If the first element had a sibling, insert the wrapper before the
    // sibling to maintain the HTML structure; otherwise, just append it
    // to the parent.
    if (sibling) {
        parent.insertBefore(this, sibling);
    } else {
        parent.appendChild(this);
    }
};

See a working demo on jsFiddle.