function closest(el, selector) {
    var matchesSelector = el.matches || el.webkitMatchesSelector || el.mozMatchesSelector || el.msMatchesSelector;

    while (el) {
        if (matchesSelector.call(el, selector)) {
            break;
        }
        el = el.parentElement;
    }
    return el;
}

这个问题，你该让jquery回答啊....
自己看下源码不就可以了

closest: function( selectors, context ) {
    var cur,
        i = 0,
        l = this.length,
        matched = [],
        pos = rneedsContext.test( selectors ) || typeof selectors !== "string" ?
            jQuery( selectors, context || this.context ) :
            0;

    for ( ; i < l; i++ ) {
        for ( cur = this[ i ]; cur && cur !== context; cur = cur.parentNode ) {

            // Always skip document fragments
            if ( cur.nodeType < 11 && ( pos ?
                pos.index( cur ) > -1 :

                // Don't pass non-elements to Sizzle
                cur.nodeType === 1 &&
                    jQuery.find.matchesSelector( cur, selectors ) ) ) {

                matched.push( cur );
                break;
            }
        }
    }

    return this.pushStack( matched.length > 1 ? jQuery.uniqueSort( matched ) : matched );
},