Adding classes to first and last columns via jQuery

gistfile1.js

for (var i = 0; i < teaserData.length; i++) {

    // Create teaser markup
    var teaser = $('<div/>').addClass('teaser');
        
    // ...
    // Constructing the teaser
    // ...

    teaser.appendTo(container);

    // Marks the first and last column by a 3 column layout
    if (teaser.is(':nth-child(3n-2)')) {
        teaser.addClass('firstColumn');
    } else if (teaser.is(':nth-child(3n)')) {
        teaser.addClass('lastColumn');
    }
    
}