Jonas Brothers bring some early 'Happiness' to BLI Summer Jam at Jones Beach

If there were any questions left about the power of the Jonas Brothers’ reunion, they were answered by the ear-splitting screams more than four hours into the BLI Summer Jam.

Though it was past 11 p.m. Friday and nine other acts had already performed, Northwell Health at Jones Beach Theater was still packed with fans, ready to scream along with Joe, Kevin and Nick Jonas.

“We’re ready to have a good time with you,” Joe said, following a huge ovation to the band’s new single “Cool.”

It’s what fans have been waiting nearly a decade to hear. The Jonas Brothers went on hiatus in 2009, following the release of their “Lines, Vines and Trying Times” album. Though they got together for a handful of shows in 2012, the planned, full-blown reunion tour never happened.

The Jonases plan to fix that this summer, with the “Happiness Begins” tour that kicks off in August. Nick called their BLI Summer Jam appearance the band’s “unofficial album release party,” as they rolled out the reggae-tinged “Only Human” from “Happiness Begins,” which will debut at No. 1 on Billboard’s album chart next week.

The Jonas Brothers went all out in their half-hour set, backed by two singers and a five-piece band. They fully embraced their boy-band past, leading fans in a scream-along to “Year 3000,” released when Nick was just 13, and “That’s Just the Way We Roll.” And they brought their outside projects into the band with Joe and Nick sharing vocals on Nick’s solo hit “Jealous” and “Cake by the Ocean” from Joe’s band DNCE.

However, it was the sleek confidence of their recent No. 1 hit “Sucker” that showed how much the Jonas Brothers have grown. It was a potent ending to more than five hours of nearly nonstop music from WBLI 106.1-FM’s upbeat pop orbit on Long Island.

Ellie Goulding charmed with her sweet hit “Love Me Like You Do,” before strapping on a guitar and rocking out to “Burn.” Mark Ronson delivered a fun DJ set of his hits, including a surprise mashup of his first hit “Ooh Wee” and the Oscar-winning “Shallow” that he co-wrote with Lady Gaga. And Why Don’t We showed why they plan to be the leading boy band for a new generation.

However, Fletcher, the New Jersey singer-songwriter who said the last time she played Jones Beach was on a stage “in a parking lot, next to a hot dog stand,” made the strongest impression of BLI’s flock of newcomers, with a great four-song set that even added a bit of Dan + Shay’s “Tequila” to her own hit “Undrunk.”

“This is the coolest experience of my life,” said Fletcher, clearly enjoying playing to a huge crowd. It won’t be her last.