Newcastle had been accused of losing concentration by Phill Neville on Match of the Day earlier in the month, but Janmaat has suggested that the former Manchester United and Everton man was wrong.
The Dutch defender believes their desire and application was evident against Arsenal, but they weren't rewarded for their efforts.
“We showed that we aren’t playing in flip flops," he said. "We are playing in football boots. Mentally we are strong. It’s a shame and we didn’t get the result.”
Newcastle were the better side for much of the second-half against Arsenal, but they couldn't turn around the deficit that they were facing.
A resolute Arsenal back-line, some excellent goalkeeping from David Ospina and a host of wasted chances from Newcastle's forwards meant that they ended up on the wrong side of a 2-1 reverse.
It was a disappointing way for the match to end, but supporters of the club will still be heartened by the fight that the Magpies showed.
Too often this season Newcastle have surrendered when falling behind, but they displayed their character on Saturday and that will be something which they can build upon in future weeks.