Answers to Exercises - Beginning JavaScript - page 624

}
var lastAdNumber = localStorage.getItem("lastAdNumber");
var nextNumber = getRandomNumber(0, ads.length);
if (lastAdNumber == null) {
lastAdNumber = nextNumber;
} else {
lastAdNumber = parseInt(lastAdNumber, 10);
while (lastAdNumber == nextNumber) {
nextNumber = getRandomNumber(0, ads.length);
}
}
localStorage.setItem("lastAdNumber", nextNumber);
document.write(ads[nextNumber]);
</script>
</body>
</html>
Save this as ch13 _ question2.html .
This solution is loosely based on similar questions in previous chapters where you have displayed
random images or messages. In this case you display a different message in the page each time the
user visits it; you'll never see the same message displayed two times in a row in the same browser.
You store the last number of the previously displayed ad in localStorage with the
key lastAdNumber . So, you retrieve that value and generate the next number with a
getRandomNumber() helper function (you know this algorithm):
var lastAdNumber = localStorage.getItem("lastAdNumber");
var nextNumber = getRandomNumber(0, ads.length);
If lastAdNumber is null , you can use the value in nextNumber :
if (lastAdNumber == null) {
lastAdNumber = nextNumber;
}
But if lastAdNumber is not null , you need to generate a random number that is not lastAdNumber .
So first, you convert lastAdNumber to a number with the parseInt() function:
else {
lastAdNumber = parseInt(lastAdNumber, 10);
while (lastAdNumber == nextNumber) {
nextNumber = getRandomNumber(0, ads.length);
}
}
Then you use a while loop to generate a unique random number. The loop iterates if lastAdNumber is
equal to nextNumber , and it continues to do so until the next number is different than lastAdNumber .
Once you have a unique next number, you store it in localStorage and display the ad in the page: