1//you can either do this:
2
3function mousePressed() {
4	//you can have any message
5  alert("I'm the best");
6}
7
8//or you can do this:
9
10function keyPressed() {
11	//you can do any key
12  if (keyCode === UP_ARROW) {
13  	//and any message
14    alert(number = random(0, 500));
15  }
16}
17
18//or you can do an (if) statement
19
20if(x > width) {
21 	alert("yay you did it");
22}

1Alert in JS doesn't return anything -- it just Displays Message

1go to inspect element, go to console. type in 
2alert("text")