King of Prussia Mall, a 2.8 million-square-foot shopping center outside Philadelphia, is in position to make a stand against online shopping.

The 50-year-old complex has more than 50 food venues and a concierge lounge. But a J.C. Penney department store closed in July, creating a hole in the anchor-store lineup. Simon Property Group is planning a mixed-used development for the 17-acre Penney site, part of a makeover that CEO David Simon has compared to Hudson Yards in Manhattan.

"I don't think people appreciate how dynamic these properties are and how they evolve over a long period of time," said Rick Sokolov, Simon's president and COO.

Retail landlords have spent billions on changes that focus on experiences that can't be found online, brokerage Jones Lang LaSalle says.

King of Prussia is the second-largest mall in the US after the Mall of America in Minnesota, according to the Directory of Major Malls.

Indiana-based Simon is spending $1 billion a year to upgrade its properties, Sokolov said.

"We're spending money to make our properties incrementally more relevant and more attractive to retailers and generating returns while we do it," he said.

Triple Five Group of Canada is building a giant "anti-mall" in northern New Jersey, just west of Manhattan.

The $5 billion complex, called American Dream, is a massive hybrid between a shopping center and amusement park.

"It will have less retail space than Mall of America and 60 percent more in entertainment offerings," Debbie Patire, Triple Five's senior VP of marketing, told philly.com. "But it really isn't about size these days. It is about the customer experience … combined with best-in-class entertainment, attractions and restaurants."

Phase 1 of the mall is under construction next to MetLife Stadium, where the NFL's Giants and Jets play.