Store variables in flash

Hi. a have same question, but with esp8266. need to save one variable after manual change and restore it from memory after reboot.

//actuation temp. this variable need to store and restore from memory
let maxTemp = 21;

buttonPlus.on('click', function() {
  maxTemp++;
   save();
});

buttonMinus.on('click', function() {
  maxTemp--;
   save();
});

thanks