So the first line you’ve left in there,
myVar = myVar++; will assign myVar to the value of myVar and ignore the ++. Instead, get rid of that line. The second line is all the test is looking for.
var myVar = 12 // sets the value of myVar
myVar // shows you the current value
myVar = myVar++ // let's try this first
myVar // again, show the current value of mVar
myVar++ // now lets try the increment operator
myVar // and one more time, let's see the current value