Mobile could represent about 12% of  e-commerce sales this holiday season as a result of higher smartphone and tablet  penetration, according to a new research note from JPMorgan analyst Doug Anmuth.  He projects e-commerce spending overall in the fourth quarter will grow 19%  across desktop and mobile, compared to 18% in the prior quarter.

Among other top holiday shopping trends,  Anmuth expects showrooming to be bigger than ever, with 37% of consumers  browsing products in physical stores, but buying online or on mobile devices at  lower prices. The online holiday shopping season begins earlier and ends later  as Web shopping on Thanksgiving Day becomes more popular.

Fourth-quarter e-commerce sales are expected  to surpass 10% of total retail spending this year excluding autos, gas and food.  JP Morgan also estimates eBay will see a 17% rise in gross merchandise volume in  the quarter, while Amazon will post a 35% revenue gain and e-commerce growth of  19%.